Low thermal stress catadioptric imaging system

ABSTRACT

An imaging system having reduced susceptibility to thermally-induced stress birefringence comprising relay optics and projection optics. One of either the relay optics or the projection optics is a reflective optical system that includes reflective optical elements, and the other is a refractive optical system having a negligible or low susceptibility to thermal stress birefringence. The refractive optical system includes: a first group of refractive lens elements located upstream from an aperture stop, and a second group of refractive lens elements located downstream from the aperture stop. The refractive lens elements in the first and second groups that are immediately adjacent to the aperture stop are fabricated using optical materials having a negligible susceptibility to thermal stress birefringence, and the other refractive lens elements in the first and second groups are fabricated using optical materials having at most a moderate susceptibility to thermal stress birefringence.

CROSS-REFERENCE TO RELATED APPLICATIONS

Reference is made to commonly assigned, co-pending U.S. patentapplication Ser. No. 12/784,520 (96201), entitled “Low thermal stressbirefringence imaging lens”, by Kurtz et al.; to commonly assigned,co-pending U.S. patent application Ser. No. 12/784,521 (96314), entitled“Designing lenses using stress birefringence performance criterion”, byBietry et al.; to commonly assigned, co-pending U.S. patent applicationSer. No. 12/784,523 (96315), entitled “Low thermal stress birefringenceimaging system”, by Kurtz et al.; and to commonly assigned, co-pendingU.S. patent application Ser. No. ______ (Docket K000684), entitled “Lowthermal stress catadioptric imaging optics”, by Kurtz et al., each ofwhich is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to optical imaging systems, and moreparticularly to imaging lens having a low sensitivity to stressbirefringence induced by the thermal load imparted by the transitinglight.

BACKGROUND OF THE INVENTION

Projection and electronic display systems are widely used to displayimage content. In the case of projection systems, whether thetraditional film based systems, or the newer electronic systems, lightfrom a light source (typically a lamp) is directed to an imagemodulation element (such as film or one or more spatial lightmodulators) that imparts image data to the transiting light.

Typically the film or light modulator arrays are then imaged to thedisplay surface or screen.

As another aspect, there is growing interest in high-quality projectionsystems that display 3 dimensional (3D) or perceived stereoscopiccontent in order to offer consumers an enhanced visual experience.Historically, stereoscopic content was projected in theaters using filmmedia, such that two sets of films are loaded to two separate projectionapparatus, one for each eye. Left- and right-eye images are thensimultaneously projected using polarized light, where one polarizationis used for the image presented to the left eye; light of the orthogonalpolarization is then used for the image presented to the right eye.Audience members wear corresponding orthogonally polarized glasses thatblock one polarized light image for each eye while transmitting theorthogonal polarized light image.

Recently, electronic or digital cinema projectors that provide stereoprojection have been commercialized. In particular, projectors based onthe Digital Light Processor (DLP) or Digital Micro-mirror Device (DMD),developed by Texas Instruments, Inc., Dallas, Tex., are used in theatresin both stereo and non-stereo versions. DLP devices are described in anumber of patents, for example U.S. Pat. No. 4,441,791, U.S. Pat. No.5,535,047 and U.S. Pat. No. 5,600,383 (all to Hornbeck).

FIG. 1A shows a simplified block diagram of a projector 100 that usesDLP spatial light modulators. A light source 50 (typically a xenon arclamp) provides polychromatic unpolarized light into a prism assembly 55,such as a Philips prism, for example. Prism assembly 55, which is shownas a Philips prism, splits the polychromatic light into red, green, andblue component wavelength bands and directs each band to a correspondingspatial light modulator (SLM) 170 r, 170 g, or 170 b. Prism assembly 55then recombines the modulated light from the spatial light modulators170 r, 170 g, and 170 b and provides this unpolarized light to animaging lens 200 for projection onto a display screen or other suitablesurface. DLP-based projectors have demonstrated the capability toprovide the necessary light throughput, contrast ratio, and color gamutfor most projection applications from desktop to large cinema.Alternately, liquid crystal devices (LCDs), which modulate light byaltering polarization states of transiting light, can be used instead ofDLP devices, with comparative benefits of higher resolution and largerdevice size, but greater difficulty in delivering contrast, contrastuniformity, and color uniformity for the projected image.

Conventional methods for forming stereoscopic images from theseSLM-based projectors (DLP or LCD) use either of two primary techniquesto distinguish between the left- and right-eye content. One less commontechnique, utilized by Dolby Laboratories, for example, uses color spaceseparation, such as described in U.S. Patent Application Publication2007/0127121 by Maximus et. al. Filters are used in the white-lightillumination system to momentarily block out portions of each of theprimary colors for a portion of the frame time. The appropriate coloradjusted stereo content that is associated with each eye is thenpresented to each modulator for the eye. The viewer wears acorresponding filter set that similarly transmits only one of the two3-color (RGB) spectral sets. Color space separation avoids problems inhandling polarized light from the projector, at the screen, and with theviewer's glasses, but light inefficiencies and the cost of the glassesare problematic.

The second method for forming stereoscopic images uses polarized light.For example U.S. Pat. No. 6,793,341, by Svardal et al. describes amethod in which the two orthogonal polarization states are provided byseparate spatial light modulators and projected simultaneously onto thescreen, which typically has properties to preserve the polarizationstates of the reflected light. The viewer wears polarized glasses withpolarization transmission axes for left and right eyes orthogonallyoriented with respect to each other. Although this arrangement offersefficient use of light, it can be an expensive configuration.

Another approach, commercialized by Real-D, Beverly Hills, Calif., usesa conventional projector modified to modulate alternate polarizationstates that are rapidly switched from one to the other. In particular,as shown in FIG. 1A, a DLP projector is modified to have a polarizer andpolarizer switching device placed in the output path of the light, suchas at a position 90 indicated by a dashed line in FIG. 1A. Thepolarization switcher is required, since DLP projectors outputmodulated, but unpolarized light. This output is unpolarized becauseunpolarized light sources (lamps) are used, and the typical DLP devicewindows are depolarizing (due to stress induced birefringence). Anachromatic polarization switcher, such as that of U.S. Pat. No.7,528,906 to Robinson et al. can be placed at position 90 after thepolarizer. A switcher of this type (the ZScreen™) alternately rotatespolarized light between two orthogonal polarization states, such aslinear polarization states, to allow the presentation of two distinctimages, one to each eye, while the user views the projected image withpolarized glasses.

Because the polarization contrast specifications for the polarizer aremodest (˜50:1) as a trade-off to boost polarizer efficiency, imagecrosstalk between the left-eye and right-eye images can occur. This cancause viewers to experience ghost images, for example such that the lefteye not only sees a bright left-eye image but a dim right-eye image.Real-D provides a variety of solutions to this problem, including theuse of real time digital pre-processing of the image content to reduceghosting in image. In particular, a digital processor applies acrosstalk model to predict potential ghosting by comparing the left- andright-eye images, and then it subtracts the predicted ghost image. U.S.Patent publication 2006/0268104 to M. Cowan et al., entitled“Ghost-compensation for improved stereoscopic projection”, expands uponthis approach. As another example, in U.S. Pat. No. 7,518,662 by Chen etal., the polarization contrast of the ZScreen switcher is improved witha tilted polarization compensator.

For a variety of reasons, including improving light efficiency,expanding color gamut, increasing light source lifetime and reducingongoing replacement costs, there is continuing impetus to replace thetraditional lamps (such xenon arc, tungsten halogen, or UHP) with solidstate light sources (such as lasers or LEDs) in projectors, whether 2-Dor 3-D. However, to date, the desire for laser-based projection systemshas been unfulfilled, in part as compact, robust, low-to-moderate cost,visible wavelength laser technologies had not emerged in acommercializable form, particularly for green and blue. With the recentemergence of blue diode lasers and compact green SHG lasers, low cost,laser based, pico-projectors from companies such as Microvision arereaching the market place.

In parallel, similar obstacles for compact high power visible laserscapable of supporting digital cinema projection have also started todisappear, as companies such as Laser Light Engines (Salem, N.H.) andNecsel (Milpitas, Calif.) have demonstrated prototype or early productlaser devices. For example, Necsel (previously known as Novalux) offersgreen (532 nm) and blue (465 nm) laser arrays, each of which provides3-5 Watts of optical output power. At these power levels, and allowingfor system efficiency losses, a modest sized projector (˜1500 lumensoutput) for a large conference room or a home theatre, can be achievedusing a single laser device per color. However, in the case of cinema,the on-screen luminance requires 10,000-40,000 lumens or 40-170 Watts ofcombined optical power (flux) incident to the screen, depending onscreen size and screen gain. In turn, allowing for internal opticalefficiency losses, this means that 40-120 Watts of optical power isrequired from the laser sources in each color channel. Presently, thesepower levels can only be accomplished by optically combining the outputof multiple laser arrays in each color channel. Eventually, the lasertechnologies may advance such that a single compact laser device candrive each color. Of course, each approach has its advantages anddisadvantages, relative to trade-offs of simplicity, cost, andsusceptibility to laser failure.

Simplistically, a digital cinema projector can be provided by replacingthe conventional lamp used for the light source 50 of FIG. 1A with amultitude of laser devices. Moreover, as lasers are inherently polarizedlight sources, more efficient 3D projection can be provided, as thepolarization switcher is used at position 90, without an accompanyingpolarizer. However, this simplistic view is unrealistic for high powerlaser-based projection applications such as for digital cinema. As justsuggested, a projection system providing 40-170 optical watts on screenis subjected to much higher light levels internally, with the highestlight levels (in flux or watts) occurring at the light sourceassemblies, and the lowest likely at the output surface of theprojection lens. Because of its spatial and temporal coherence, laserlight focuses into smaller volumes with higher power densities thanlight beams from incoherent (lamp) sources, even when flux levels arecomparable. The highest internal power densities occur in places wherethe light is concentrated, such as at an integrating bar, the spatiallight modulators, aperture stops, or intermediate images. Of course,these high light levels can bring accompanying thermal issues, asillumination light, imaging light, or even stray light encountersinternal surfaces or materials. There are already numerous problemscaused by the intense light in conventional lamp based systems, some ofwhich will only be amplified in laser-based systems. For example, in alamp-based system, the input aperture of the integrating bar, whichreceives high intensity focused light and surrounding stray light fromthe lamp, is typically surrounded with an air-cooled heat sink assembly.As another example, in digital cinema projection systems, the spatiallight modulators are typically cooled with circulating chilled water.

At such high light levels, the intense light (and particularly residualUV light) can also impact the performance or reliability of materials,including optical adhesives, cements, or polymers used in prismelements, doublets, or liquid crystal devices. As a result, thesematerials must be chosen carefully to avoid induced degradation fromthermal or chemical changes. Likewise, induced mechanical stresses frommismatched coefficients of thermal expansion of the optical elements ortheir mounting assemblies must also be minimized or managed to avoidstress, deformation, or breakage.

As one particularly subtle effect, which effects polarization basedprojection systems, including those for 3D projection, small portions ofthe high light intensity light can be absorbed by the optical materials,thereby inducing stress birefringence with the elements. That in turncan change the polarization orientations of the transiting light,thereby impacting image contrast, image contrast uniformity, coloruniformity or other attributes that reduce the perceived on-screen imagequality.

In the case of spatial light modulator devices, and liquid crystal onsilicon (LCOS) devices in particular, a problem can occur where theintense light causes thermal loading and stress birefringence in thecounter electrode substrate, which is internal to the device itself. Togive further context, FIG. 1B illustrates a prior art projector 101, inwhich incident illumination light beams 140 are directed into respectivemodulation optical systems 80 for each color, which are projectorsub-systems that comprise a polarization beamsplitter 60 (also known asa polarization prism), a polarization compensator 360, and a spatiallight modulator 170 g, 170 b or 170 r. The modulated beams from themodulation optical systems 80 are combined using an X-prism 65, anddirected to projection lens 270 for projection onto a display screen(not shown). Typically, the polarization behavior and properties ofthese components within the modulation optical system 80 determines theon-screen polarization contrast provided by the projector 101.

The counter electrode substrate (not shown) is a thin plate of opticalglass that is laid parallel to the silicon substrate within LCD spatiallight modulators 170 g, 170 b and 170 r. Liquid crystal materials, aswell as pixel structures formed into (or on) the silicon, then fill thethin gap between these substrates. The counter electrode substrate iscoated with a patterned transparent electrode (typically of ITO), toenable electric fields to be applied between the substrates to controlthe orientations of the liquid crystal molecules on a pixel wise basis.

This structure works well at low light intensities, such that thepolarization orientations commanded by the pixels are maintained as thelight transits the counter electrode substrate, and the resultingpolarized image light can then encounter downstream polarization optics,such as polarization beam splitters, analyzers, or switches, with thepolarized image light having the intended orientations. However, underhigh light intensities, the portion of the light transiting the counterelectrode substrate that is absorbed can cause sufficient internalheating to induce stress birefringence, which in turn alterspolarization orientations.

In recognition of this problem, U.S. Pat. No. 5,576,854 to Schmidt etal., proposes a method for identifying optimal glasses that can be usedto fabricate the counter electrode substrate of an LCOS panel. Inparticular, they proposed a figure of merit M for identifying candidateglasses given by the product:

M=ρEκ  (1)

where ρ is the coefficient of thermal expansion (CTE), κ is the stressoptical coefficient, and E is the modulus of elasticity (E). Schmidt etal. identified two glasses as particularly valuable candidates; SchottSF-57 for its unusually low stress optical coefficient, and fused silicafor its unusually low coefficient of thermal expansion. According toSchmidt et al., in the case of fused silica, heating causes minimalexpansion of the glass, which in turn cause little thermally-inducedstress. In the case of SF-57, the thermal stress coefficient itself isvery low, which means little direct translation of heat into stressbirefringence. As alluded to earlier, a similar problem presently existswith the cover glass windows for DLP modulators; but as these deviceshave generally not been used to modulate intense polarized light with anexpectation of maintaining polarization states, neither the glassselection nor the glass mounting design were undertaken with the goal ofminimizing stress birefringence.

The relationship of glass selection and thermal stress birefringence inprojection displays is also explored in the article “Thermal StressBirefringence in LCOS Projection Displays”, by R. Cline et al., whichwas published in Displays, Vol. 23, pp. 151-159, 2002. This article isconcerned with identifying glasses appropriate for use in polarizationbeamsplitters 60 (FIG. 1B) or Philips prism assemblies 55 (FIG. 1A) inprojection display systems. In particular, the authors introduce anexpanded figure of merit for assessing candidate glasses that includesnot only the coefficient of thermal expansion (ρ), the stress opticalcoefficient (κ), and the modulus of elasticity (E), but also the glassthermal conductivity (K), light absorption (α), and Poisson's ratio (μ):

$\begin{matrix}{M = \frac{{\alpha\rho}\; E\; \kappa}{K\left( {1 - \mu} \right)}} & (2)\end{matrix}$

Cline et al. propose that only Schott SF-57, Ohara PBH56, and fusedsilica can be used in prisms for high power polarization sensitiveprojectors (1000+ lumens), while a wider range of glasses, includingSchott SK5 or Schott BK7, can be used for prisms in low power (≦500lumen) projectors.

In contrast, in U.S. Pat. No. 7,357,511 to Aastuen et al., the inventorssuggest that the glasses proposed by Cline et al., for satisfactory lowstress birefringence (such as Schott SK5 or Schott BK7) are actuallyinadequate, and that contrast degradation from these alternate glassesis actually too large. Aastuen et al. then propose an alternatemodulation optical system 80 where the polarization contrast of thepolarization beamsplitter 60 can be improved relative to stressbirefringence in the glass comprising the prism, includingthermally-induced stress birefringence, by providing a polarizationcompensator 360 between the polarization beamsplitter 60 and the spatiallight modulator 170 (see FIG. 1B). They provide evidence that apolarization compensator 360 having a quarter wave of retardance canprovide sufficient compensation for stress birefringence such that theprism glass choice is no longer limited to low stress opticalcoefficient (κ) glasses, such as Schott SF-57.

It is also noted that unwanted birefringence has caused image qualityproblems in fields outside of the projection space, including in thearea of micro-lithography. For example, in U.S. Pat. No. 6,785,051 toAllan et al., describes a refractive/reflective imaging system directedat 200 nm UV microlithography. In that spectral range, the very smallselection of available optical materials is dominated by crystallinematerials such as calcium fluoride (CaF₂) that exhibit significantintrinsic birefringence. In order to reduce the accumulativebirefringence or polarization state changes in the optics, Allan et al.provide one or more corrective optical elements (optical plates orbeam-splitters) which are also fabricated from the same type ofintrinsically birefringent materials. In this case, a correctivephotoelastic birefringence is provided by an externally applied stressor strain (from tensile, compressive or shear stress) which was impartedto the corrective element by mechanical fixturing, a piezoelectricactuator, a thermal element or other stress inducer.

Likewise, U.S. Pat. No. 6,879,379 to Brunotte et al., also discloses aUV microlithographic imaging system using lens elements comprisingintrinsically birefringent materials such as CaF₂ and a correctiveelement. The intrinsic birefringence imparts unwanted polarizationrotation effects with position and angle. In this case, the correctiveelement is an optical plate or lens which is located proximate to anaperture stop, and which is also made from CaF₂. Mechanical stresses arethen applied in a pulsed fashion using piezoelectric actuators, so as toimpart stress birefringence to the element that compensates for theangle dependent polarization effects caused by the intrinsicbirefringence.

While interesting, the solutions of Brunotte et al. ('379) and Allan etal. ('051) apply to imaging systems using a limited set of intrinsicallybirefringent materials. By comparison, the solutions provided by Schmidtet al. ('854), Cline et al., and Aastuen et al. ('511) were developed inthe context of lamp-based projection systems that were targeted for lowpower applications, but are potentially extendable to digital cinema.However, these solutions are narrowly targeted at the optical components(cover glasses and prisms) within the modulation optical systems 80 fora projector 101.

In laser projection systems the localized light intensities and powerdensities can be appreciably higher as compared to white-light systems,due to the coherence or focusing power of the laser light, and thermaleffects can be induced throughout an optical system. In extreme cases,optical self-focusing effects in non-linear optical materials can causeoptical damage or breakdown.

In the case of laser-based digital cinema projectors, while permanentdamage mechanisms such as self-focusing are likely not germane, otherthermal effects such as thermally-induced stress birefringence canaffect optical elements, including components other than those residingin the modulation optical subsystems, such as the prism assemblies,spatial light modulators, or cover plates or counter electrode substratetherein. In particular, the design and use of imaging lens assemblies,which comprise a complex multitude of lens elements, and which aretasked to image intense laser light while not being subject tothermally-induced stress birefringence and resulting polarizationeffects, is a concern, particularly at the digital cinema power levels,which are much higher than managed previously for stress birefringence.As is known to those skilled in the lens design arts, an imaging lensassembly utilizes an arrangement of non-planar lens elements, whosematerials, thicknesses, curvatures, and relative placements arecarefully designed to provide the desired image quality, relative toaberrations and diffraction. However, the added complexity of furthercontrolling thermally-induced stress birefringence, relative to thedesign of an imaging lens system and the constituent lens elements, is aproblem that is neither taught nor anticipated in the prior art.

SUMMARY OF THE INVENTION

The present invention represents an imaging system having reducedsusceptibility to thermally-induced stress birefringence, for projectingan image of an object plane onto a display surface, comprising:

imaging optics including relay optics that image the object plane ontoan intermediate image plane and projection optics that image theintermediate image plane onto the display surface;

wherein one of either the relay optics or the projection optics is areflective optical system that includes reflective optical elements, andthe other of the relay optics or the projection optics is a refractiveoptical system having a negligible or low susceptibility to thermalstress birefringence, the refractive optical system including:

-   -   a first group of refractive lens elements located upstream from        an aperture stop; and    -   a second group of refractive lens elements located downstream        from the aperture stop;

wherein the refractive lens elements in the first and second groups ofrefractive lens elements that are immediately adjacent to the aperturestop are fabricated using optical materials having a negligiblesusceptibility to thermal stress birefringence as characterized by athermal stress birefringence metric, and the other refractive lenselements in the first and second groups of refractive lens elements,that are not the refractive lens elements immediately adjacent to theaperture stop, are fabricated using optical materials having at most amoderate susceptibility to thermal stress birefringence as characterizedby the thermal stress birefringence metric.

It has the advantage that the performance of the imaging system when itis used to produce images using polarized light will not besignificantly affected by thermal changes resulting from the absorptionof the imaging light.

It has the further advantage that the imaging system can be used forstereoscopic projection systems without producing objectionablecross-talk between the left- and right-eye images due tostress-birefringence-induced depolarization.

It has the additional advantage that the reduced birefringencesusceptibility is achieved while simultaneously achieving an acceptableimage quality level.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more readily understood from the detaileddescription of exemplary embodiments presented below considered inconjunction with the attached drawings, of which:

FIGS. 1A and 1B illustrate portions of prior art digital projectionsystems;

FIG. 2 illustrates the overall system architecture for a projectorincorporating an imaging lens of the present invention;

FIGS. 3A and 3B illustrate exemplary light source assemblies, includingmultiple laser devices and a laser combiner, as used in a projectionsystem of the present invention;

FIG. 4 illustrates the general construction of an imaging lens of thepresent invention, comprising both a relay lens and a projection lens;

FIG. 5A depicts a prior art double gauss type imaging lens;

FIG. 5B depicts a glass chart, plotting optical glasses relative totheir index of refraction and Abbe number;

FIG. 6A depicts the optical design of a first exemplary projection lens,having moderate thermal stress susceptibility and good image quality;

FIG. 6B depicts the optical performance of the projection lens of FIG.6A using MTF plots;

FIG. 6C depicts the optical design of a first exemplary relay lens,having moderate thermal stress susceptibility and good image quality;

FIG. 6D depicts the optical performance of the relay lens of FIG. 6Cusing MTF plots;

FIGS. 7A and 7B illustrate contour and cross-sectional beam profiles oflight beams emergent from the laser combining assembly;

FIGS. 7C and 7D illustrate contour and cross-sectional illuminationprofiles incident to the spatial light modulators;

FIG. 7E illustrates a light intensity distribution in telecentric spacefollowing the integrating bar;

FIG. 7F illustrates a light intensity distribution near the aperturestop of the relay lens;

FIG. 7G illustrates a light intensity distribution near the aperturestop of the projection lens;

FIGS. 8A-8D are tables showing optical glass properties;

FIG. 9A is a table summarizing the thermal stress birefringenceperformance of the first exemplary projection lens of FIG. 6A;

FIG. 9B is a table summarizing the thermal stress birefringenceperformance of the first exemplary relay lens of FIG. 6C;

FIG. 10A depicts the optical design of a second exemplary projectionlens, having low thermal stress susceptibility and poor image quality;

FIG. 10B depicts the optical performance of the second exemplaryprojection lens of FIG. 10A using MTF plots;

FIG. 10C depicts the optical design of a second exemplary relay lens,having low thermal stress susceptibility and poor image quality;

FIG. 10D depicts the optical performance of the second exemplary relaylens of FIG. 10C using MTF plots;

FIG. 11A is a table summarizing the thermal stress birefringenceperformance of the second exemplary projection lens of FIG. 10A;

FIG. 11B is a table summarizing the thermal stress birefringenceperformance of the second exemplary relay lens of FIG. 10C;

FIG. 12A depicts the optical designs of a third exemplary projectionlens, having low thermal stress susceptibility and good image quality;

FIG. 12B depicts the optical performance of the third exemplaryprojection lens of FIG. 12A using MTF plots;

FIG. 12C depicts the optical design of a third exemplary relay lens,having low thermal stress susceptibility and good image quality;

FIG. 12D depicts the optical performance of the third exemplary relaylens of FIG. 12C using MTF plots;

FIG. 13A is a table summarizing the thermal stress birefringenceperformance of the third exemplary projection lens of FIG. 12A;

FIG. 13B is a table summarizing the thermal stress birefringenceperformance of the third exemplary relay lens of FIG. 12C;

FIG. 14A is a table specifying the lens design parameters for the thirdexemplary projection lens of FIG. 12A;

FIG. 14B is a table specifying the lens design parameters for the thirdexemplary relay lens of FIG. 12C;

FIG. 15 is a flowchart illustrating a method for designing imaging lenswith reduced thermal stress birefringence susceptibility according to anembodiment of the present invention; and

FIG. 16 is a flowchart illustrating another method for designing imaginglens with reduced thermal stress birefringence susceptibility accordingto an embodiment of the present invention;

FIG. 17 depicts a method for reducing axial color while using aprojection lens of the present invention;

FIG. 18 depicts an alternate embodiment of a projector incorporating aprojection lens having reduced susceptibility to thermally-inducedstress birefringence in combination with an Offner relay in accordancewith the present invention;

FIG. 19 depicts an exemplary embodiment of catadioptric projectionoptics having reduced susceptibility to thermally-induced stressbirefringence in accordance with the present invention;

FIGS. 20A and 20B are tables specifying the lens design parameters forthe exemplary catadioptric projection optics of FIG. 19; and

FIG. 21 depicts another exemplary embodiment of catadioptric projectionoptics having reduced susceptibility to thermally-induced stressbirefringence in accordance with the present invention.

It is to be understood that the attached drawings are for purposes ofillustrating the concepts of the invention and may not be to scale.

DETAILED DESCRIPTION OF THE INVENTION

The present description is directed in particular to elements formingpart of, or cooperating more directly with, apparatus in accordance withthe invention. It is to be understood that elements not specificallyshown or described may take various forms well known to those skilled inthe art.

The invention is inclusive of combinations of the embodiments describedherein. References to “a particular embodiment” and the like refer tofeatures that are present in at least one embodiment of the invention.Separate references to “an embodiment” or “particular embodiments” orthe like do not necessarily refer to the same embodiment or embodiments;however, such embodiments are not mutually exclusive, unless soindicated or as are readily apparent to one of skill in the art. The useof singular or plural in referring to the “method” or “methods” and thelike is not limiting. It should be noted that, unless otherwiseexplicitly noted or required by context, the word “or” is used in thisdisclosure in a non-exclusive sense.

In order to better understand the present invention, it is instructiveto describe the overall context within which apparatus and methods ofthe present invention can be operable. The schematic diagram of FIG. 2shows a basic arrangement for a projector 102 that is used in a numberof embodiments of the present invention. Three illumination assemblies110 r, 110 g and 110 b are shown, each providing one of the primary Red,Green, or Blue (RGB) colors from a respective light source assembly 115.The light source assemblies 115 include one or more light sources, whichin particular are laser light source devices. (The laser light sourcedevices are not shown in FIG. 2, but are shown in representative fashionin FIGS. 3A and 3B.)

FIG. 3A shows one approach for combining light from multiple laserarrays 120 and 120′ to form a laser combining assembly 125, which is asub-portion of a light source assembly 115 (FIG. 2). High powersemiconductor (or solid state) laser arrays of several Watts or moreoptical output have been readily available in the red and infrared (IR)for many years. These lasers typically comprise a single row of singlemode by multimode laser emitters 122. However, high power compact greenand blue laser arrays are only now becoming available. Thus far, thepreviously mentioned lasers from Necsel, which are a type of IR pumped,frequency doubled, VECSEL (Vertical Extended Cavity Surface-EmittingLaser) laser, have shown particular promise. Due largely to heat andpackaging problems for critical components, the currently availablepre-commercialized devices have a constrained architecture that providestwo parallel rows laser emitters 122 (24 emitters per row).

While it is optically beneficial to closely arrange the output lightbeams from the constituent laser arrays, it is also desirable tomechanically separate the laser arrays 120 from each other to reducethermal crosstalk and concentrated thermal loads. It can also bedesirable to separate the light source assemblies 115 (FIG. 2), as wellas the electronic delivery and connection and the associated heat, fromthe thermally-sensitive, optical projection system, to allow optimalperformance of the projection engine. In FIG. 3A, one or moreinterspersed mirrors 160 can be used to place the optical axis ofadditional laser arrays 120′ in line with laser array 120 to provide amultitude of light beams 140, each comprised of a plurality ofindividual light beams 140′, directed together toward an illuminationlens 150 having an associated aperture 130, which is a component of therespective illumination assembly 110 r, 110 g and 110 b (FIG. 2).

FIG. 3B then depicts a portion of an exemplary illumination assembly 110according to an alternate embodiment. The illumination assembly 110includes a light source assembly 115 for a given color that comprisestwo laser combining assemblies 125. Using such an arrangement, the poweroutput can be increased, to support larger screens with higher screenlumen requirements. In this example, each of the two laser combiningassemblies 125 utilizes a faceted prism 127 with window facets andreflecting facets (which work by total internal reflectance) on eachside, to redirect light beams 140 from laser emitters 122 in laserarrays 120 down a common optical path. The output light from the twolaser combining assemblies 125 is directed by illumination beam combiner135 down a common optical path towards the other illumination optics, asrepresented by illumination lenses 150 and light integrator 155. Theillumination beam combiner 135 can combine the light beams 140 by avariety of approaches, including spectrally (for the case where thelaser arrays 120 in the laser combining assemblies 125 are clustered onopposite sides of a central wavelength), spatially, or by polarization.One path can have an optional half wave plate 137.

Considering FIGS. 2 and 3B in combination, each illumination assembly110 r, 110 g and 110 b, typically includes one or more illuminationlenses 150, a light integrator 155 (such as a fly's eye integrator orintegrating bar, for example) that shapes and direct the transitinglight beams, and further illumination lenses 150 and mirrors 160, thattogether direct illumination light along an optical axis 145 to anassociated spatial light modulator 170. For example, incoming light fromthe light source assembly 115 can be directed into a light integrator155 using an illumination lens 150. The resulting homogenized lightfills an output aperture of the light integrator 155. The outputaperture is then re-imaged to an area in an optical plane to which thespatial light modulator 170 is aligned. The illumination lenses 150 andthe light integrator 155 can be fabricated using fused silica in orderto reduce any polarization degradation effects that might occur frominduced stress birefringence in these elements.

The spatial light modulators 170 can be micro-electromechanical systems(MEMS) devices, such as a DLP or some other type of reflective MEMSdevice, including any of the types of MEMS modulators that modulatelight by reflection or by diffraction. The spatial light modulators 170can also be LCD-type devices or of other technologies. In the case ofDLP-type devices, modulation provides “On” state or image light that isdirected to the display surface, and “Off” state light that is directedinto a beam dump (not shown). In the case that projector 102 switchesthe orientations of polarized light sources to drive stereo imageprojection (3D), then “polarization state neutral” modulator devices arepreferred. In particular, spatial light modulators 170 are preferredthat do not modulate light at each pixel by modulating the polarizationstate of the pixel, such that any change to the polarization state ofthe incident light for any pixel is inadvertent and small. This meansthe spatial light modulators 170 will preferably modulate the incidentlight identically on a pixel wise basis, regardless of the polarizationstate of the incident light. Therefore, it is presumed that audiencemembers are wearing polarization sensitive glasses so as to view thestereoscopic or 3D images. It should also be understood that theprojector 102 can provide conventional images that are perceived as twodimensional.

Unlike projectors that provide 3D imagery with a polarization switchingaccessory added externally to the projector, in this projector 102,illuminating light from illumination assemblies 110 is intended to bepolarized. In particular, the light sources are arranged to provide acommon polarization state, known as “s-polarized” or “p-polarized” inthe language of the field. The illumination assemblies 110 can include avariety of optics, including wave plates or polarizers (not shown) toalign, preserve, or accentuate the native polarization states of thelight sources. The illumination assemblies 110 can also include apolarization switch 139, which can be electro-optically orelectro-mechanically actuated, to change the polarization state of thelight illuminating spatial light modulators 170 to s-polarized,p-polarized, or other polarization states (such as left or rightcircular) that are useful for 3D image projection. As a result, it ispreferred that the various illumination components, including theillumination lenses 150 and the light integrator 155 be polarizationpreserving. The light path within each illumination assembly 110 r, 110g and 110 b follows the same basic pattern, although there can bedifferences to accommodate differences in light source (laser)properties from one color channel compared to another. Each illuminationassembly 110 can have its own polarization switch 139, which can beoperated in synchronized fashion with each other, or a commonpolarization switch 139 can be used for multiple color channels.

As shown in FIG. 2, illumination light is directed onto spatial lightmodulators 170 by redirection with one or more mirrors 160. Modulatedimage light, bearing image data imparted into the transiting light bythe addressed pixels of the spatial light modulators 170 is combined totraverse a common optical path passing through imaging lens 200 and ontoa display surface 190 (such as a projection screen). In the illustratedembodiment, a dichroic combiner 165 comprises a first combiner 166 and asecond combiner 167, each of which is a dichroic element havingappropriate thin film optical coatings that selectively transmits orreflects light according to its wavelength. As this projector 102 isdesigned to provide 3-D image content using internal modulation of theoptical polarization states, the dichroic combiner 165 and the imaginglens 200 should also be polarization neutral, such that little or nodifferences in efficiency, polarization contrast, or image quality areinduced by these elements. Likewise, display surface 190 is preferably apolarization-preserving screen.

It should be understood that mirrors 160 need not lie in the plane ofthe optical system. Thus the mirror 160 in the optical path for thegreen channel can be out of plane, and not obstructing light passing toprojection lens 270, as might be otherwise implied by FIG. 2.Additionally, while dichroic combiner 165 is shown as a pair of tiltedglass plates, other exemplary constructions can be used, includingX-prisms 65 (FIG. 1B), V-prisms, or Philips (or Plumbicon) type prisms(FIG. 1A). In other embodiments, mirrors 160 can also be provided in theform of prisms, such as the widely used TIR (total internal reflection)prism that is often used in combination with the Philips prism and DLPdevices.

In FIG. 2, imaging lens 200 is depicted as a multi-element assemblycomprising multiple lens elements 205 that images spatial lightmodulators 170 r, 170 g and 170 b at their respective object planesdirectly onto an image plane (display surface 190) at high magnification(typically 100×-400×).

FIG. 4 expands on the design of the imaging lens 200, depicting anembodiment where the imaging lens 200 comprises two portions, a relaylens 250 and a projection lens 270, each of which comprise a multitudeof lens elements 205 operating at finite conjugates and assembled into alens housing 240. For example, relay lens 250 is designed to collect andimage F/6 light from the object planes of the spatial light modulators170 to form a real aerial intermediate image 260 at a correspondingimage plane. This intermediate image 260 is then the object plane forprojection lens 270, which provides a magnified image of theintermediate image to distant image plane (display surface 190), whichis the nominal image plane, within an allowed depth of focus. That is,the spatial light modulators 170 are image conjugate to the intermediateimage 260, which in turn is image conjugate to the display surface 190.

The relay lens 250 preferably provides a long working distance 245, of150-200 mm or more, to provide clearance for the dichroic combiner 165and the mirrors 160 (FIG. 2) in the vicinity of the spatial lightmodulators 170. An exemplary relay lens 250 images the spatial lightmodulators 170 with a lateral magnification of 1× or slightly larger,providing a real intermediate image 260 whose size is comparable to theimage area of a frame of 35 mm movie film. As a result, projection lens270 can potentially be a conventional projection lens designed toproject 35 mm film images, such as the projection lenses currentlymanufactured by Schneider-Kreuznach of Bad Kreuznach, Germany.

While the imaging lens 200 depicted in FIG. 4 can seem more complicatedthan that of FIG. 2, in practice the relay lens 250 and the projectionlens 270 of FIG. 4 are easier to design and produce, and therefore areof lower cost, then the equivalent unified imaging lens 200 present incommercial DLP cinema projectors today. In part this is because it iseasier to provide a long working distance 245 while working near 1×magnification than when working at high magnification. Additionally,this approach enables laser de-speckling, by providing a convenientlocation for insertion of a despeckler 180, such as a moving diffuser,at or near the intermediate image 260. In this system, the despeckler180 is preferably a lenslet array comprising sparsely placed lensletshaving dimensions the size of one or more imaged pixels. With thedespeckler 180 in operation, the projection lens 270 is preferably afaster lens (˜F/3) than the relay lens 250.

Given this background, it is noted that an experimental laser basedprojector 102, having the general configuration depicted in FIGS. 2, 3A,and 3B, but utilizing the imaging lens 200 of FIG. 4, with the relaylens 250, intermediate image 260, and projection lens 270 has beenassembled and tested by the inventors. In operation, one prototypeversion of the system exhibited a loss of polarization contrast, suchthat the blue channel polarization contrast was ˜400:1 or greater whenthe projector was operated for low luminance output (at 3000 lumens forexample), but was degraded to ˜100:1 when the projector is operated atlevels above ˜6000 lumens without the despeckler 180, and to ˜150:1 withthe despeckler 180 in operation. In particular, a polarization change orloss was most apparent for blue imagery as blue light experiences higherlight absorption levels. The polarization change caused perceptiblecrosstalk or ghosts for projected stereoscopic images by viewers wearingpolarization discriminating glasses. While this crosstalk can beremedied using the ghost correction digital post processing methodsoffered by Real-D, the present invention provides a preferable solutionwhere internal lens design corrections can be used to eliminate the needfor such correction.

In particular, as the polarization contrast degrades with light level,this is consistent with the finding that the problem originates withthermally-induced stress birefringence. As will become apparent, asolution is provided which involves the selective use of optical glassesin the design of the relay lens 250 and the projection lens 270. Both ofthese lenses, as depicted in the embodiment of FIG. 4, are generallydouble Gauss type lenses. The basic double Gauss lens dates back to the1800's, with C. F. Gauss and then Alvan Clark developing the originalform. First, C. F. Gauss improved upon a Fraunhofer telescope objectiveby adding a meniscus lens to its single convex and concave lens design.Alvan Clark then took two of these lenses and placed them back to backto obtain the double Gauss design. Paul Rudolph improved upon thisfurther, with the use of cemented doublets to correct for chromaticaberrations, as described in U.S. Pat. No. 583,336. A double Gauss lensconsists of two Gauss lenses back to back, forming two groups of lenselements 208 positioned about the aperture stop 230, in which the lenselements of the two groups may be identical, as demonstrated by Rudolph.

FIG. 5A depicts a prior art double gauss type imaging lens 200 thatimages an object surface 195 along an optical axis 145 to a distantimage plane (not shown). In a common basic form, it consists of an innerpair of negative lens elements 206 b of flint type glasses positionedabout the aperture stop 230, and outer pairings of positive lenselements 206 a consisting of crown type glasses. In this example, thelens element designs on either side of the aperture stop 230 (the Gausslens assemblies) are not identical.

The significance of crown and flint glasses is illustrated by the glasschart 210 shown in FIG. 5B, which is an “Abbe-diagram” plotting thewidely recognized glass chart data published by Schott Glass, Inc. Crownglasses 215 have low dispersion (indicated by a high Abbe numberν_(d)>50) and generally have a low refractive index (n_(d)), while flintglasses 217 have comparatively high dispersions (low Abbe numberν_(d)<50) and generally have higher refractive indices. Weak-flints orweak-crowns, with intermediate dispersion characteristics, are foundnear the intermediate Abbe number, ν_(d)˜50. In FIG. 5A, the lenselements are also identified as either crown lens elements 220 which ismade using a crown glass 215 or a flint lens elements 222 which is madefrom a flint glass 217.

The general symmetry of the double Gauss design approach and thesplitting of the optical power into many elements reduces the opticalaberrations imparted by the lens system. It forms the basis for manycamera lenses in use today, especially the wide-aperture standard lensesused on 35 mm and other small-format cameras. A completely symmetricaldesign is free of coma, distortion and TCA (transverse chromaticaberration or lateral color). There are many variations of the design,with extra lens elements added, or with asymmetrical designs about theaperture stop 230, with the symmetry sacrificed to achieve other goals.As examples, U.S. Pat. No. 4,704,011 to R. Mercado describes a doubleGauss type photographic objective, while U.S. Pat. No. 6,795,255 to W.Reinecke et al. describes a double Gauss type movie lens for projectingimages from film, and commonly assigned U.S. Pat. No. 5,172,275 to D.DeJager describes a complex double Gauss type lens applicable to motionpicture film scanning

FIG. 6A illustrates the design of a first exemplary projection lens 270having a double Gauss lens design with two groups of lens elements 208similar to the projection lens 270 within the imaging lens 200illustrated in FIG. 4. The projection lens 270 has two inner negativelens elements 402 and 403 positioned about an aperture stop 230 that areflint lens elements 222, negative lens elements 402 being fabricatedusing Schott SF1 glass and negative lens elements 403 being fabricatedusing SF2 glass. The projection lens 270 also has 4 outer positive lenselements 400, 401, 404 and 405, all of which are crown lens elements220. The largest positive lens element 405 on the far right isfabricated using Ohara S-BSM-10, while the other positive lens elements400, 401 and 404 are all fabricated using Ohara S-LAM-60, from OharaGlass Inc. These glasses have close Schott glass equivalents, which areN-SK10 and N-LAF35, respectively. Optical rays 235 are shown tracing thepath of light rays through the projection lens 270.

FIG. 6B shows a modulation transfer function (MTF) plot 300 whichdepicts the broad spectrum MTF performance of the exemplary projectionlens 270 in FIG. 6A. While not diffraction limited, it can be seen thatthe MTF averages ˜65% at 50 cy/mm over the imaged field.

FIG. 6C depicts the design of a first exemplary relay lens 250, which isa stretched version of a double Gauss lens design, similar to the relaylens 250 illustrated in FIG. 4. The relay lens 250, which has two groupsof lens elements 208 plus a field lens (lens element 416) is used toform an intermediate image 260 (at an image plane) of the spatial lightmodulator 170 (which is positioned at an object plane). This lens designincludes a pair of negative lens elements 412 and 413 positioned aroundthe aperture stop 230 that are flint lens elements 222 fabricated usingSchott SF4 glass. The relay lens 250 also includes a collection ofpositive lens elements 410, 411, 414 and 415 that are crown lenselements 220. The positive lens elements 410 and 411 are fabricatedusing Ohara S-BAL35. The positive lens element 414 is fabricated usingOhara S-LAM54, and the positive lens element 415 is fabricated usingOhara S-NSL3. The relay lens 250 also includes a positive field lenselement 416 positioned near the intermediate image 260, also fabricatedusing Ohara S-LAM54. Dichroic combiner 165 is also shown represented bya non-tilted planar element.

FIG. 6D depicts the optical performance of the relay lens 250 from FIG.6C with MTF plot 300. The optical performance is nearly diffractionlimited, with an average MTF of ˜79% at 50 cy/mm over the image field.

As noted previously, a conventional imaging lens 200, comprising thecombination of a conventional relay lens 250 (i.e., that shown in FIG.6C) and a conventional projection lens 270 (i.e., that shown in FIG.6A), was found to experience a drop in blue light polarization imagecontrast to ˜150:1 or less and blue image ghosting, when transmittinglaser light to the display surface 190 for moderately high screenbrightness levels (6,000-11,000 lumens), compared to ˜400:1 polarizationcontrast at low screen lumen levels. Separate measurements of thepolarization contrast of the output light when the imaging lensassemblies were not present showed little change with luminance levels,thus confirming that the relay lens 250 and projection lens 270 were theprimary culprits. In particular, these measurements showed that the bluechannel (465 nm) polarization contrast degraded to ˜100:1, while thegreen and red contrast levels remained higher (˜300:1). Taken together,this data strongly suggested that the polarization contrast wasdegrading due to thermally-induced stress birefringence effects, whichwould only worsen at yet higher power levels. As most optical glasses(except most notably fused silica) experience higher absorption in theblue, stress birefringence can be induced more readily by blue lightabsorption. As a result, stress birefringence can affect image qualityin the different color channels to varying extents.

Thermally-induced stress birefringence is the change in refractive index(Δn(λ,T)) induced in the glass with temperature (T), and it functionallydepends on many parameters, including the wavelength λ, the absorptionof the glass (α), the optical stress susceptibility of the glass (κ),and the coefficient of thermal expansion (ρ). It also depends on thespatio-temporal distribution of light intensity (irradiance) or powerdensity (for example, in units of W/mm², lumens/mm², or lux) for thelight transiting the glass.

FIGS. 7A-7F illustrate exemplary light intensity distributions that cantransit the optics of the projector 102 depicted in FIGS. 2, 3A, 3B and4. As depicted in FIGS. 3A and 3B, the laser combining assemblies 125can produce a multitude of output light beams 140, which combine to fillor partially fill an aperture 130. Because of the structure of the laserarrays 120, which can have one or more spaced arrays of offset laseremitters 122, a multitude of individual light beams 140′ within adepicted light beam 140 are obtained. Combining beams from multiplelaser arrays 120 increases the multitude of individual light beams 140′,many of which have traversed different optical path lengths in reachingthe aperture 130. As the individual light beams 140′ propagate, theymerge and overlap into each other. Depending on the position in theoptical system, the relative optical path lengths traversed by thevarious individual light beams 140′, and the use of light homogenizers,integrators, or diffusers, some light beams 140′ or images of laserarrays 120 or combinations thereof, may be more distinguishable thenother individual light beams 140′ or combinations thereof. The netresult is that cross-sections of the light intensity distributions canshow varying amounts of structure, depending on the location within theoptical system.

To expand upon this last point, FIG. 7A illustrates a modeled lightintensity distribution 320 near the aperture 130 of a light sourceassembly 115, in which the individual light beams 140′ from theinnermost laser arrays 120 are more defocused than the light beams fromthe outermost laser arrays. FIG. 7B illustrates two cross-sectionalprofiles 322 through the light intensity distribution 320 of FIG. 7A atslice positions 321. These light intensity distributions 320 are highlystructured, rather than spatially uniform, as the multitude of lightbeams 140′ are only partially overlapped with each other, and the amountof overlap differs from center to edge.

Patterns similar to that shown in FIG. 7A re-occur throughout theoptical system. For example, in the angular far field of the lightintegrator 155 (FIG. 3B), a similarly patterned light intensitydistribution 326 emerges as shown in FIG. 7E, although multiple bounceswithin an integrating bar have made it more complicated. Per the designintent, the integrating bar produces a nominally uniform light intensitydistributions 323 at its output face as shown in the contour view ofFIG. 7C. FIG. 7D shows corresponding cross-sectional profiles 325corresponding to the slice positions 324 shown in FIG. 7C.

The light intensity distribution 323 at the output face of theintegrating bar is then re-imaged to illuminate the spatial lightmodulator 170 of that color channel. Relay lens 250 (FIG. 4) re-imagesthe spatial light modulators 170 to form a combined white light image(depending on image content) as real aerial intermediate image 260,which the projection lens 270 subsequently re-images onto the displaysurface 190. At, or near to, the aperture stops 230 of the relay lens250 and the projection lens 270, highly structured light intensitypatterns, echoing the light intensity distribution 326 of FIG. 7E, canappear, depending on the optical diffusion or angular averaging appliedin the system. For example, FIG. 7F illustrates an exemplary modeledlight intensity distribution 327 near the aperture stop 230 of the relaylens 250, while FIG. 7G illustrates an exemplary modeled light intensitydistribution 328 near the aperture stop 230 of the projection lens 270(without the inclusion of a despeckler 180).

It is noted that the use of despeckler 180 can cause the propagatinglight to be angular broadened, or spatio-temporally averaged within theprojection lens, or both. As a result, the micro-structure of the lightintensity distribution 320 at or near the aperture stop 230 of theprojection lens 270 may not be as sharply defined, nor temporallystationary. Nonetheless, the lens elements 205 within the relay lens 250and the projection lens 270 can experience transiting light having highoptical power densities, particularly near the aperture stops 230 wherethe light intensity distributions 326 and 327 can have the lightconcentrated in numerous peaks in the micro-structure of the lightintensity distributions. These concentrations of light energy can causethermally-induced optical stress birefringence throughout these lensassemblies, but particularly in lens elements 205 near the aperturestops 230.

As further background to understanding birefringence, it is known thatthe propagation of light can be described by wave equations, includingEq. (3) which describes a plane polarized wave ψ(x,t) as a function ofthe distance x and the time t, where A(x,t) is the amplitude function,and φ(x,t) is the phase of the disturbance:

Ψ(x,t)=A(x,t)e ^(iφ() x,t)  (3)

The phase of the propagating wave can be written as:

φ(x,t)=ωt−kx=ω(t−x/v)  (4)

where, ω is the rate of change of phase with time, k is the rate ofchange of phase with distance, and v is wave velocity. The value ω isalso known as the angular frequency, where ω=2π/ν, and the value k isalso known as the propagation number, where k=2π/λ_(o). The frequency νand the wavelength λ_(o) of light in free space are related by the speedof light, c=λ_(o)/ν.

As light enters and traverses an optical material, it can experience avariable reflectivity and phase change Δφ, depending on the angle ofincidence, the polarization orientation of the incident light relativeto the media, the refractive index n of the optical material, and thethickness of the material. The Fresnel equations, which model surfacereflectance or transmission, affect the amplitude term of Eq. (3). Therefractive index in a material or medium is basically the ratio of thespeed of light in vacuum c to the speed v in the medium (n=c/v).Substituting into Eq. (4) puts the phase in terms of the refractiveindex:

φ(x,t)=ωt−(2πn/λ ₀)x  (5)

Even in an isotropic material of constant refractive index, adifferential phase change Δφ can occur across different beam propagationangles, as the optical path length (d/n) in the material changes fromone angle to another, where d is the thickness of the material. However,in the case of a complex structure, such as a birefringent material, adifferential amount of rotation and ellipticity can be induced in thetransiting light. Thus, differential phase changes Δφ can occurdepending on the angle and polarization states of the incident light;(Δφ=Δφ_(sp)=φ_(p)−φ_(p)). In this nomenclature, “s” and “p” denotewhether the electric field vector of the light oscillatesperpendicularly to the plane of incidence (s-polarization) to a surface,or p-polarized light, where the electric field vector oscillatesparallel to the plane of incidence over the entire range of incidenceangles.

Birefringent materials are non-istropic materials with respect topolarization. That is, birefringence is a directional variation ofrefractive index (Δn_(sp)=n_(s)−n_(p)=n_(x)−n_(y)), and it can beprovided by intrinsic material properties, by form-birefringentsub-wavelength structures, or by induced mechanical stresses. Retardanceis the resulting phase change Δφ expressed as distance (R=Δφλ), wherethe phase change Δφ(x,d,λ)=2πd(Δn/λ). For example a π/2 (or 90°) phasechange Δφ can be provided by a properly oriented birefringent element(wave plate) having a quarter wave λ/4 of retardance, which, at 550 nm,equals ˜138 nm retardance.

In particular, the application of uniform temperature produces changesin the mechanical stress σ of optical components, such as cementedelements, due to mismatches in coefficients of thermal expansion betweenthe elements and/or the mounting materials. Temperature gradients, ascan be associated with light intensity distribution micro-structuresdiscussed previously, also induce stress in single, homogeneouselements. Either residual stresses imparted during manufacturing orfabrication processes, or states of stress from pressure, inertial, orvibratory loads can cause birefringence in optical elements. In essence,the effect of stress, whether mechanically-induced or residual, is tochange the index of refraction of the optical material.

In this case, the application of sufficient temperatures within therelay and projection lens elements of projector 102, particularly inlocalized regions, causes thermally-induced optical stressbirefringence. The induced birefringence difference is directlyproportional to the induced difference in the principal mechanicalstresses or stress tensors, Δσ_(1,2)=(σ₁(x,y,z)−σ₂(x,y,z)) between twospatial proximate locations, as given by equation (4);

Δn=Δn _(1,2)(x,y,z)=κΔσ_(1,2)  (6)

where κ is the stress optical coefficient of the material, given inunits of mm²/N and the induced mechanical stress σ is typically given inN/mm² (or MPa). The induced stress σ, or equivalently the induced stressbirefringence Δn, can be written in terms of the material temperaturechange induced by heating:

Δn≈ρEΔTκ/(1−μ)  (7)

where ΔT is the induced temperature change. In this equation, ρ is thecoefficient of thermal expansion (CTE), which for room temperature rangeglass, is typically given in units of 10⁻⁶/^(o)K. The variable E is theYoung's modulus, which is a measure of the stiffness of an isotropicelastic material, and which is often given in units of N/mm². Poisson'sratio, the unit-less variable μ, is a metric for the orthogonal responseof a material to a strain (stretching or contraction).

The induced temperature change ΔT, can be related to the lightabsorption. The amplitude function A(x,t) can be expanded to show itsdependence on light absorption α:

A(x,t)=A(x)=A ₀ e ^(−αx/2)  (8)

where A₀ is the initial amplitude. This leads to Beer's Law thatdescribes the exponential nature of light absorption:

I(x)=I ₀ e ^(−αx)  (9)

where I(x) is the light intensity (or irradiance) in units of W/m², andI₀ is the initial light intensity.

The volume heat generation Q(x), in W/m³, from the light absorbed withina thickness x of a material, as a function of the light intensity I(x),and the internal transmittance t_(i) or the absorption coefficient α,is:

Q(x)=I(x)(1−t _(i))/x=I(x)(1−e ^(−αx))/x  (10)

The law of Heat Conduction, or Fourier's Law, in a one-dimensional formis given by q_(x)=−K dT/dx, where q_(x) is the local heat flux, in W/M²,and K is the thermal conductivity (in W·m⁻¹·K⁻¹). It is noted that whileheat transfer and balance (steady state) depends on heat conduction,convection, and radiation, for an optical structure, conduction is oftenthe most important factor in determining temperature gradients orchanges. Also, the thermal conductivity K of most materials, includingglass, is fairly constant over a broad range or temperatures. Fourier'sLaw can be integrated to derive a change in temperature ΔT affecting anarea from light absorption:

ΔT≈I ₀ L×α/4K  (11)

where L is the axial thickness of the optical element

Substituting Eq. (11) into Eq. (7) links the stress inducedbirefringence to the incident light intensity and the materialabsorption:

$\begin{matrix}{{\Delta \; n}\; \approx {I_{0}L \times \frac{\rho \; {\kappa\alpha}\; E}{4{K\left( {1 - \mu} \right)}}}} & (12)\end{matrix}$

This equation suggests several figure of merits or metrics which areuseful to the present invention. The first thermal stress birefringencemetric:

$\begin{matrix}{M_{1} = \frac{\rho \; {\kappa\alpha}\; E}{K\left( {1 - \mu} \right)}} & (13)\end{matrix}$

is a materials (glass) only stress birefringence metric which can beuseful in selecting candidate glasses, and which has units of W⁻¹. Asecond intensity-weighted thermal stress birefringence metric:

$\begin{matrix}{M_{2}\; = {{I_{0}L\; M_{1}} = {I_{0}L\frac{\rho \; {\kappa\alpha}\; E}{K\left( {1 - \mu} \right)}}}} & (14)\end{matrix}$

is valuable because it factors in the light intensity (i.e., opticalpower density) incident (I₀) to the optical element and the axialthickness (L) of the optical element.

In Eq. (12), the distance x is a localized width within the material,relating to the fact that the intensity given in Eq. (9) and the changein temperature given in Eq. (11) cause localized changes in stressbirefringence Δn as a function of position. Including x in metrics M₁ orM₂ would add little insight relative to glass selection for differentlens elements.

An intermediate thermal stress birefringence metric, M₃=M₁·L, suggeststhat thinner lens elements (smaller thickness L) can tolerate highervalues of M₁. However, as will be seen, the variations in M₁ and I₀ tendto dominate the selection and design process.

The second metric M₂ factors in the power densities present in the lenselements 205, which can vary through the lens assemblies of the relaylens 250 and the projection lens 270. As the light intensitydistributions in the lenses (see FIGS. 7A-7E) can evidence,micro-structures with the highest light intensities occur in regionsproximate to the aperture stops 230. As a result, the inventors havediscovered that glass selection is most critical for lens elements inthese regions. Therefore, the intensity-weighted thermal stressbirefringence metric M₂ has been found to be helpful, provided that peaklight intensities (I₀) can be reasonably estimated.

The inventors used finite element analysis (FEA) to thermal-mechanicallymodel the temperature induced stress from light absorption in theprojection lens 270 of FIG. 6A. This modeling indicated that the opticalflux levels transiting the negative lens elements 402 and 403 nearestthe aperture stop 230 of the projection lens 270 can be 8× (or more)higher than the light levels transiting the outermost positive lenselements 400 and 405, causing localized temperature changes of ΔT˜5° C.These localized temperature changes in turn cause mechanical stress, andthus birefringent refractive index changes Δn.

With this background, The tables given in FIG. 8A-8D provides data on anassortment of modern glasses by name, along with key parametersincluding their refractive indices n_(d), Abbe number ν_(d), internaltransmission t_(i), absorption α (where α=−ln(t_(i))/x), stress opticalcoefficient κ, thermal conductivity K, coefficient of thermal expansion(CTE) ρ, Poisson's ratio (μ), Young's modulus (E), and the glass-onlyfigure of merit M₁. The absorption coefficient values (α) werecalculated using internal transmittance values at λ=460 nm. Althoughother assessment wavelengths can be used, most glasses experience anincrease in internal absorptance in the UV-to-blue spectral range, andthus have increased stress birefringence potential compared to green orred spectral ranges. FIG. 8A shows various glass properties for acollection of low stress birefringence susceptibility glasses. FIG. 8Bshows the same properties for a collection of moderate and high stressbirefringence susceptibility glasses. FIGS. 8C and 8D show thecalculated values for the thermal stress birefringence metrics M₁′, M₁″and M₁, for the glasses in FIGS. 8A and 8B, respectively.

The tables of FIGS. 8A-8D show a subset of the glasses (<15%) availablein the combined Schott and Ohara glass catalogs, which feature adisproportionate share of glasses that have advantageous properties forlow thermally-induced stress birefringence. One of the most commonglasses, Schott BK-7 glass, has a good low value for the figure of meritM₁ (˜0.51×10⁻⁶ W⁻¹), while the new lead free glasses (such as N-SF2 andN-SF4) tend to have higher M₁ values than the original glasses (SF-2 andSF-4) that they replace. The tables of FIGS. 8A-8D includes only Schottor Ohara optical glasses, but alternative or equivalent glasses fromother manufacturers can be analyzed and used in design appropriately.

The glasses in the tables of FIG. 8A-8D are also identified as towhether they are considered to be crowns or flints. By this glassmetric, the best potential glasses, relative to minimizingthermally-induced stress birefringence continue to be fused silica(represented by Schott Lithosil-Q) and Schott SF-57 (or its equivalents,such as Ohara PBH56), as they have very low or negligible thermal stressbirefringence metric values (M₁<0.1×10⁻⁶ W⁻¹). These glasses are thesame ones suggested by Schmidt et al., and others. In examining thistable, which spans nearly the full range of available glasses, it isnoted that the glass only thermal stress birefringence metric M₁ variesover a range of 25,000:1 from the best glass (Lithosil; M₁˜0.001×10⁻⁶W⁻¹) to the worst glass (Ohara S-NPH2; M₁˜28.5×10⁻⁶ W⁻¹). Notably, thereis a ˜8× or more jump in the M₁ values from the negligible stressbirefringence susceptibility glass group (M₁≈0.001×10⁻⁶ W⁻¹ to 0.05×10⁻⁶W⁻¹) that contains fused silica, SF-57, and PBH56, to the best of thelow stress birefringence susceptibility glasses, such as Schott LLF1 orOhara S-NSL36 (M₁≈0.36×10⁻⁶ W⁻¹ to 0.46×10⁻⁶ W⁻¹).

The Tables of FIGS. 8C-8D also include two constituent figures of meritM₁′ and M₁″. The first constituent figure of merit M₁′=ρκα, and accountsonly for the dominant parameters: the coefficient of thermal expansion(ρ), the stress optical coefficient (κ), and the absorption coefficient(α), whose values vary widely, by ˜30×, ˜200×, and ˜950×, respectively,amongst the different glasses. M₁′ has units of 10⁻¹² mm/KN. Relative tothe first constituent figure of merit M₁′, fused silica has the smallestvalue as its absorption and CTE are both very low, while SF-57 and PBH56do well because their stress optical coefficients are low. Likewise, theworst glasses relative to the glass-only thermal stress birefringencemetric M₁, Schott N-SF4 and Ohara S-NPH4, are the worst relative to thefirst constituent figure of merit M₁′ metric as well, with values6,000-12,000 greater than Lithosil fused silica.

However, for the purposes of choosing glasses for lens design,accurately discriminating among the glasses with low or moderate stressbirefringence susceptibility is important. It is noted that severalmaterials properties, namely Poisson's ratio μ, Young's modulus E, andthe thermal conductivity K all individually vary in a limited range of˜2-2.5×, and thus have secondary impact on the glass-only thermal stressbirefringence metric M₁. However, in making glass selection decisionsfor lens design, these secondary factors can be important. Thus, thetables of FIGS. 8C-8D also shows a secondary constituent metric M₁″ forjust these terms, where M₁″=E/(K(1−μ)) and M₁=M₁′·M₁″·M1″ has units of10⁶ NK/W.

In looking at the data column for the secondary constituent metric M₁″,it is noted that fused silica (Lithosil) does well again, because it'sPoisson's ratio μ and thermal conductivity K have advantageous values.But glasses such as Ohara S-NSL36, Ohara S-NSL-3, Schott LF-5, andSchott LLF1 are also comparatively advantaged relative to other glasses,as their M₁″ values are 2-3×lower than the most penalized glasses. Thus,using M₁′ as a surrogate for M₁ can give an incorrect indication of thestress sensitivity relative to the glasses with low or moderate M₁values, and potentially lead to poor glass choices during lens design.

The accompanying tables shown in FIGS. 9A and 9B provide estimates forthe intensity-weighted thermal stress birefringence metric M₂ for thelens elements in the first exemplary designs, projection lens 270 ofFIG. 6A and the relay lens 250 of FIG. 6C, respectively. In each ofthese tables, the lens elements are identified with the lens elementpart numbers from FIG. 6A. In both tables, values for the glass-onlythermal stress birefringence metric M₁ are given for each of the lenselements, corresponding to the values given in the tables of FIGS.8A-8D. An aggregate M₁ value is also given for each lens assembly. Inestimating the intensity-weighted thermal stress birefringence metricM₂, axial thickness values (L) are used for the lens elements, althoughalternate values, such as the average lens thickness of the traversedclear aperture of a given lens element, could be used instead. Thetables also include values for the normalized power loads on eachelement, based on modeled estimated optical power loads in W/mm² or lux.The intensity-weighted thermal stress birefringence metric M₂ is thenestimated for each lens element using the normalized power load values.An aggregate M₂ value is also given for each lens assembly.

The first exemplary projection lens design of FIGS. 6A-6B was designedwith priority for image quality, and with thermally-induced stressbirefringence not ignored, but treated as a secondary consideration.Thus, the table of FIG. 9A shows a glass set comprising glasses whichare all solidly in the moderate range for the glass-only thermal stressbirefringence metric (M₁˜1.0×10⁻⁶ W⁻¹ on average). The glasses used inthis design do not include any glasses having negligible M₁ values (suchas PBH56 or fused silica), or any glasses having very high M₁ values(such as Schott N-SF2 or N-SF4). As the various elements have comparablethermal stress birefringence metric M₁ values, and as the highestapplied power densities occur at lens elements 400-402, which areclosest to the spatial light modulator 170, the aggregateintensity-weighted thermal stress birefringence metric M₂ is dominatedby lens elements 400-402.

FIG. 10A illustrates the design of a second exemplary projection lens270. Like the first exemplary projection lens of FIG. 6A, this lensgenerally has the classic double Gauss form with two groups of lenselements 208, with four inner negative lens elements 422, 423, 424 and425 about the aperture stop 230, and four outer positive lens elements420, 421, 426 and 427. This lens was designed with a priority onreducing thermally stress birefringence, therefore all of the lenselements are either fused silica lens elements 428 or Ohara PBH56 lenselements 429. In particular, the four inner flint glass lens elements422, 423, 424 and 425 are fabricated using the very low stress OharaPBH56 glass, and the four outer crown glass lens elements 420, 421, 426and 427 are all fabricated using the lowest absorption crown glass,fused silica.

In considering the tables of FIGS. 8A-8D, it is noted that the SchottSF57 glasses and Ohara S-FSL-5 have glass-only thermal stressbirefringence metric M₁ values that are ˜½ that of Ohara PBH56. However,other considerations influence the design choices. In particular, theSchott SF57 types are in rare supply, whereas PBH56 is readilyobtainable in fairly large quantities. Furthermore, the Ohara S-FSL5glass is very expensive (about 13× that of BK7), and is not verydesirable relative to high susceptibility to staining

The table of FIG. 11A shows thermal stress birefringence metric valuesfor the second exemplary projection lens of FIG. 10A. It can be seenthat this configuration has a greatly reduced M₁ figure of merit that is˜35× lower in value than the first exemplary projection lens of FIG. 6A(compared to the table of FIG. 9A). However, as can be seen from the MTFplot 300 in FIG. 10B, the image quality has comparatively suffered, eventhough the second exemplary projection lens of FIG. 10A has eight lenselements instead of six. In particular, as previously discussed withrespect to FIG. 6B, the first exemplary projection lens provides ˜65%MTF at 50 cy/mm over the imaged field. By comparison, the secondexemplary projection lens of FIG. 10A provides ˜68% MTF on-axis, butaverages an unacceptable 32% MTF off-axis. The off-axis MTF is only halfof what was achieved with the first exemplary projection lens.

Similarly, FIG. 10C illustrates a second exemplary relay lens 250 whichwas designed with high prioritization to reducing susceptibility tothermally-induced stress birefringence. In particular, all the lenselements 430-436 have negligible M₁ values, and are either fused silicalens elements 437 or SF-57 lens elements 438. This lens has the samegeneral configuration of the relay lens 250 of FIG. 6C, except that itresembles a double Gauss type lens even more weakly. However, like theprojection lens 270 of FIG. 10A, the two innermost lens elements 432 and433 positioned about the aperture stop 230 are negative lensesfabricated using a low stress flint glass (SF-57 in this case). Theouter lens elements 430, 431, 434 and 435 are fabricated using the lowabsorption crown glass fused silica. Additionally, a field lens element436 positioned near the intermediate image 260 is fabricated usingSF-57.

FIG. 11B shows a table giving the values of the thermal stressbirefringence metrics M₁ and M₂ for the second exemplary relay lens ofFIG. 10C. Comparing them to the values given in FIG. 9B, it can be seenthat they are much lower (˜90×) than the first exemplary relay lens ofFIG. 6C, indicating that the expected susceptibility tothermally-induced stress birefringence would be greatly reduced.However, as demonstrated by the corresponding MTF plot 300 of FIG. 10D,the image quality of the second exemplary relay lens of FIG. 10C hasalso suffered relative to the first exemplary relay lens of FIG. 6C. Inparticular, whereas FIG. 6D showed ˜79% MTF at 50 cy/mm over the imagedfield for the first exemplary relay lens, FIG. 10D shows that the secondexemplary relay lens provides ˜71% MTF on-axis, and an average of only˜56% MTF off-axis.

These results suggest that a balanced design approach, giving comparablepriority to attaining both good image quality and low thermally-inducedstress birefringence, is advantageous to the designs of both theprojection lens and the relay lens. FIG. 12A illustrates the design of athird exemplary projection lens 270, which differs from its predecessorsin that a balanced design emphasis was given to both good image qualityand low thermally-induced stress birefringence. Like the first andsecond exemplary projection lenses shown in FIGS. 6A and 10A, thisprojection lens by appearance also has the classic double Gauss formwith two groups of lens elements 208, except that the larger negativemeniscus lens element 442 to the left of the aperture stop 230, is acrown fused silica lens element 446 instead of a flint (PBH56), whichgoes against the classical form of this lens type. The smaller negativemeniscus lens element 443 is a flint glass PBH56 lens element 447.However, the other lens elements, the four outer crown lens elements440, 441, 444 and 445 are Ohara S-LAL18 lens elements 448, rather thanfused silica or PBH56 lens elements.

In undertaking the design of this lens system, the use of a high-index,weak-crown glass, such as Ohara S-LAL18, was useful for these outerelements to provide an improved MTF performance relative to the secondexemplary projection lens of FIG. 10A, while providing improved thermalstress birefringence performance relative to the first exemplaryprojection lens of FIG. 6A. Amongst the candidate high-index, weak-crownglasses, Ohara S-LAL18 glass has the lowest glass-only thermal stressbirefringence metric M₁ value available (M₁=0.726×10⁻⁶ W⁻¹), compared toother such glasses, like Ohara S-LAL8 (M₁=1.15×10⁻⁶ W⁻¹), Ohara S-LAL54(M₁=1.31×10⁻⁶ W⁻¹), or Ohara S-LAL61 (M₁=1.56×10⁻⁶ W⁻¹), or high-index,weak-flint glasses like Ohara S-LAH66 (M₁=1.09×10⁻⁶ W⁻¹).

FIG. 12B depicts the MTF plot 300 for the third exemplary projectionlens 270 of FIG. 12A. The MTF averages ˜59% at 50 cy/mm at all fieldpoints. While the image quality is not quite as good as the firstexemplary projection lens (see FIG. 6B), it is much better than theperformance provided by the second exemplary projection lens (see FIG.10B).

Accompanying tables FIGS. 13A and 13B are provided for these thirdexemplary projection lens and relay lens designs, which give calculatedvalues for the thermal stress birefringence metrics M₁ and M₂. Thesetables can be compared to those for the two alternate designs for theprojection lens 270 and relay lens 250, given in FIGS. 9A-9B and FIGS.11A-11B, to explore the different trade-offs with respect to reducingstress birefringence versus providing enhanced image quality.

The table in FIG. 13A shows that the aggregate glass-only thermal stressbirefringence metric has a value M₁=2.95×10⁻⁶ W⁻¹, which is ˜2× smallerthan the value for the first exemplary lens of FIG. 6A (see FIG. 9A).This table also shows that the intensity-weighted thermal stressbirefringence metric M₂, which factors in lens element thickness and theincident light intensity, is even larger, approaching a 3× improvement.While the innermost lens elements 442 and 443 experience the highestthermal loads, their M₁ values are so low that the M₂ values remain loweven with the incident light intensity factored in. As a result, minimalcontrast loss is expected from these elements. While the outer lenselements 440, 441, 444 and 445 have higher M₁ values, their power loadsare much more modest, and their M₂ values are muted.

Comparing the data of FIG. 13A to that of FIG. 9A suggests that furtheropportunities to improve the lens design relative to stressbirefringence, particularly with respect to lens element 444. If thislens element could be fabricated using a glass with a lower M₁ value,the susceptibility to stress birefringence could be measurably improved.While using fused silica or PBH56 would certainly help for stressbirefringence, their dispersion properties would have a negative impacton the image quality. As an alternative, a glass near the middle of theglass chart (FIG. 5B), such as a weak-crown or weak-flint, that has bothintermediate dispersion and low stress birefringence characteristics canbe used for this element, to help balance color aberrations while alsoreducing stress birefringence.

As one example, a low-index (n_(d)=1.517) weak-crown glass (ν_(d)=52.4),such as Ohara S-NSL36, which has a smaller glass-only thermal stressbirefringence metric (M₁=0.463×10⁻⁶ W⁻¹) than does the high-index(n_(d)=1.729) weak-crown (ν_(d)=54.7) Ohara S-LAL18 (M₁=0.726×10⁻⁶ W⁻¹),can be used instead. This can be helpful, potentially reducing thestress birefringence susceptibility per M₂ by 25% compared to thecurrent third exemplary projection lens. Likewise, other neighboringglasses like Ohara S-BAL-11 (a low-index crown, n_(d)=1.572, ν_(d)=57.7,M₁=0.529×10⁻⁶ W⁻¹), Schott LLF-1 (a low-index weak-flint, n_(d)=1.548,ν_(d)=45.8, M₁=0.375×10⁻⁶ W⁻¹) or Schott LF5 (a low-index flint,n_(d)=1.581, ν_(d)=40.9, M₁=0.453×10⁻⁶ W⁻¹), all with relatively lowglass-only figure of merit values (M₁<0.53×10⁻⁶ W⁻¹) can also be usedadvantageously. However, as these glasses have lower refractive indicesthan does Ohara S-LAL18, the shape of lens element 444, or any lenselements that such a change was applied to, would likely become moresevere to deliver the same optical power, which in turn can introducemore aberrations. Understandably such substitutions cannot be donewithout incurring further design optimizations, including perhapscorrective glass changes for other elements. For example, another lenselement may be required between lens elements 444 and 445 (FIG. 12A).

In considering FIGS. 8, 11A and 13A, a threshold is suggested forchoosing glasses for lens design where the stress birefringencesusceptibility must be low or negligible. In particular, applying aglass selection limit restricting choices to glasses having low thermalstress birefringence metric values (e.g., M₁≦0.80×10⁻⁶ W⁻¹) is useful inmany cases. This limit includes not only the negligible M₁ glasses likefused silica and PBH56, but also BK7 (the most commonly used glass),together with middle-of-the-glass-chart low-index weak-flints or crowns(such as LLF1, S-NSL36) and other similar glasses (LF5, S-BAL11). Italso includes high-index, middle-of-the-glass-chart glasses (S-LAL18 andSLAL7 (M₁=0.736×10⁻⁶ W⁻¹), a high-index flint (SF6), a middle-indexflint (F2) and two glasses with useful partial dispersioncharacteristics for color correction (N-PK52A and N-FK51A). Certainly,the development of new optical glasses, resident in the middle of theglass chart with high (n_(d)>1.70) or low (n_(d)<1.60) refractiveindices, but with yet lower stress birefringence susceptibility(M₁˜0.2×10⁻⁶ W⁻¹, for example) would help this class of design problemssignificantly.

For lens designs where thermally-induced stress birefringence is a highpriority, the inventors have found that advantaged results can beobtained if the lens elements nearest the aperture stop 230 (such aslens elements 442 and 443 in FIG. 12A) are made using glasses withnegligible thermal stress birefringence metric M₁ values (e.g.,M₁≦0.1×10⁻⁶ W⁻¹). Other lens elements having small clear apertures(radial size), or experiencing high power densities (typically in thenext grouping of lens elements outward from the aperture stop 230 suchas lens elements 441 and 444 in FIG. 12A), or a field lens near a smallobject (such as lens element 456 of FIG. 12C) can also benefit from theuse of these same negligible M₁ glasses. However, for cases where imagequality optimization motivates other glass choices, then opening up theglass choices to include the low M₁ value glasses (e.g., 0.1×10⁻⁶W⁻¹≦M₁≦0.8×10⁻⁶ W⁻¹) can help significantly. For lens elementsexperiencing even less power density exposure, the glass selection listcan be relaxed further, for example to include additional moderate M₁value glasses (e.g., 0.8×10⁻⁶%≦M₁≦1.6×10⁻⁶ W⁻¹). As this thresholdexpands the list to include several high-index flint glasses (such asSF1, SF2, or SF4) and high-index, middle-of-the-glass-chart glasses(such as LAH-66, LAM60, LAL54, and LAL61), the lens design latitude isopened up significantly. In some cases, these same moderate M₁ glasses(0.8×10⁻⁶ W⁻¹≦M₁≦1.6×10⁻⁶ W⁻¹) can be used for more interior lenselements that experience higher power densities if it is found that thiswould provide a significant improvement in image quality., However, inorder to keep thermal stress birefringence levels low, the thickness (L)of such lens elements should generally be kept small (a few millimeters,for example). Using the more restrictive low M₁ threshold (M₁≦0.80×10⁻⁶W⁻¹) eliminates more than 85% of the glasses in the glass chart fromconsideration, while using the less restrictive moderate M₁ threshold(M₁≦1.60×10⁻⁶ W⁻¹) eliminates about 60% of the glasses (relative to thecurrent Schott and Ohara optical glass catalogs). Obviously, a lensdesigner can choose to select glasses where M₁>1.60×10⁻⁶ W⁻¹ in order tomeet image quality requirements, particularly for lens elementsexperiencing a low power density, however this will come at the cost ofincreased thermal stress birefringence susceptibility.

The prescription for the third exemplary projection lens 270, shown inFIG. 12A, is provided in the table of FIG. 14A, with the data for radii(lens shape or curvature), thicknesses, and materials included. All thelens surfaces have spherical, rather than aspheric, toric, orcylindrical profiles. Potentially the use of rotationally symmetricaspheric surfaces on one or more lens elements can provide even greaterdesign freedom, such that more lens elements can be made using lowthermal stress birefringence susceptibility glasses (e.g., M₁≦0.80×10⁻⁶W⁻¹ instead of M₁≦1.6×10⁻⁶ W⁻¹) while maintaining image quality.

Parameters, such as aspheric coefficients, are commonly used to describeaspheric surfaces in lens design programs such as Code V and Zemax andare described in optical texts such as “Lens Design Fundamentals,” by R.Kingslake (Academic Press, New York, 1978). Aspheric surfaces usuallyenable improvements (reduced magnitudes) to the monochromaticaberrations of a lens system. However, the use of aspheric surfacesprofiles can indirectly improve color aberrations as well byredistributing optical power in a lens system. Such surfaces may alsoyield solutions that depart from the traditional double gauss lens formdescribed earlier. Thus, the use of aspheric surfaces can enable analternate design for an imaging lens or lens system, including theprojection lens 270 of FIG. 12A, potentially providing both better imagequality performance and a better selection of glasses for low thermalstress birefringence susceptibility. For example, the use of one or moreaspheric surfaces in the design of the third exemplary projection lenscould enable lens element 444, which according to its M₂ value,experiences high power loading, to be switched to the low stress OharaS-NSL36 glass from the higher stress Ohara S-LAL18 glass, without theneed to the aberration compensating lens element proposed earlier.

FIG. 12C illustrates a third exemplary relay lens 250 which was designedwith a balanced prioritization relative to achieving both low stressbirefringence susceptibility and good image quality. This thirdexemplary relay lens has the same general configuration of the first andsecond exemplary relay lenses of FIGS. 6C and 10C, although it resemblesa classic double Gauss type lens more closely than does the FIG. 10Clens. In this lens system, the two innermost lens elements 452 and 453positioned about the aperture stop 230 are negative PBH56 lens elements457 fabricated using the low-stress flint glass PBH56. The field lenselement 456 near the image plane or intermediate image 260 is also aPBH56 lens element 457. Lens element 451 is a low-absorption crown glassfused silica lens element 459. The other lens elements 450, 454 and 455are Ohara S-LAL18 lens elements 458, which have a relatively low M₁value (M₁=0.726×10⁻⁶ W⁻¹). The prescription for the third exemplaryrelay lens 250, shown in FIG. 12C, is provided in the table of FIG. 14B,with the data for radii, thicknesses, and materials included.

The table given in FIG. 13B shows the thermal stress birefringencemetric values for the third exemplary relay lens of FIG. 12C. Inconsidering the data of FIGS. 9B, 11B and 13B, it can be seen that thethermal stress birefringence metrics M₁ and M₂ for the second exemplaryrelay lens (FIG. 11B) are much lower (˜90×) compared to the firstexemplary relay lens (FIG. 9B), whereas the third exemplary relay lens(FIG. 13B) has thermal stress birefringence metrics M₁ and M₂ that areonly 2-3× lower compared to the first exemplary relay lens. (FIG. 9B).However, the smaller improvements in the thermal stress birefringencesusceptibility is offset by better image quality performance.

FIG. 12D shows an MTF plot 300 characterizing the imaging qualityperformance for the third exemplary relay lens of FIG. 12C. As per thebalanced design intent, the image quality of this third exemplary relaylens is substantially improved relative to the second exemplary relaylens (see FIG. 10D), having ˜79% MTF at 50 cy/mm. The third exemplaryrelay lens provides nearly diffraction limited performance for all fieldpositions, much like the first exemplary relay lens (see FIG. 6D).

In examining the thermal stress birefringence susceptibility data inFIG. 13B for the third exemplary relay lens of FIG. 12C 250, it is seenthat lens element 454 is the dominant contributor to theintensity-weighted thermal stress birefringence metric M₂. This suggeststhat changing lens element 454 from SLAL-18 (M₁=0.726×10⁻⁶ W⁻¹) to alower stress glass, such as Ohara S-NSL36 (M₁=0.462×10⁻⁶ W⁻¹), wouldsignificantly reduce the thermal stress birefringence susceptibility. Ofcourse, as this glass has a lower refractive index, the lens would haveto be redesigned to compensate for the loss of optical power, whilenominally maintaining the image quality. Additional lens elements,aspheric surfaces, or other corrective approaches can be employed toaccomplish this.

The lens designs prescribed in FIGS. 14A and 14B, and shown in FIGS. 12Aand 12C, were fabricated, assembled, and tested in the projector 102(FIG. 2) of the present invention. This projector used light sourceassemblies 115 comprising multiple laser arrays 120 and combiningoptics, in general accordance with the design principle associated withFIGS. 2, 3A and 3B. This projector also used the basic imaging lensarchitecture of FIG. 4, with the third exemplary projection and relaylenses of FIGS. 12A and 12C substituted for the first exemplaryprojection and relay lenses of FIGS. 6A and 6C.

Significantly improved levels of polarization contrast were observed inan operational projector providing ˜11,000 screen lumens when using thethird exemplary projection and relay lenses of FIGS. 12A and 12C, with˜200:1 stabilized polarization contrast without a despeckler 180, and˜250:1 polarization contrast with an operating despeckler 180. Asdiscussed earlier, it is expected that even further improvements can berealized by making additional adjustments to the lens designs.

In considering the prior discussion, it is noted that lens designerstypically guide their efforts using a lens design program merit functionto combine differentially weighted terms that express their designintent. For example, a multi-factorial lens design merit function cancontain terms for focal length, aberration correction, working distance,glass selection, and numerous other parameters. However, at present,stress birefringence is not available as a design control parameter orterm available for lens design merit functions in common optical designprograms such as Code V™ (Optical Research Associates, Pasadena, Calif.)or Zemax™ (Zemax Inc., Bellevue, Wash.). Moreover, many of theconstituent factors on which the glass-only thermal stress birefringencemetric M₁ depends (such as Poisson's ratio (μ) or the stress opticalcoefficient (κ)) are also not directly available as sourced data in thedatabases of the lens design programs. As a result, in order to reducestress birefringence while optimizing image quality, the lens designerwill generally need to be guided less by automated computer processorcalculations than is typical today.

In some cases, lens design programs may have capabilities that wouldenable a lens designer to import the additional optical materials datainto the program databases, and then supply the M₁ and M₂ equations ascalculative and weighted parameters for a lens design program meritfunction to provide a method for more fully automating the design of animaging lens having a reduced susceptibility to thermally-induced stressbirefringence.

The merit function should generally include one or more image qualityperformance terms and at least one term related to a thermally-inducedstress birefringence performance term computed using a thermal stressbirefringence metric. The lens design can then be automaticallyoptimized by optimizing the merit function. In this way, the programwill be enabled to automatically balance the thermal stressbirefringence susceptibility and the image quality performance, as wellas any other relevant lens attributes.

An example of a lens design merit function Q that can be used to guidethe lens optimization process is:

$\begin{matrix}{Q = {{w_{a}{\sum\limits_{n}^{\;}{w_{A}A_{n}}}} + {w_{b}{\sum\limits_{m}^{\;}{w_{B}G_{m}}}} + {w_{c}{\sum\limits_{i}^{\;}{w_{C}M_{2,i}}}}}} & (15)\end{matrix}$

where A_(n) are aberration or image quality correction terms (forexample, for spherical aberration, astigmatism, MTF, PSF (point spreadfunction) or encircled energy), G_(m) are terms for geometrical limitsor attributes (for example, for working distances, angular limits,collected numerical aperture (NA) or F-number (F/#), field of view,magnification, focal length, track length, lens element thicknesslimits, or depth of focus) and M_(2,i) are terms for theintensity-weighted thermal stress birefringence metric for the i^(th)lens element. The weighting factors w_(a), w_(b), w_(c), W_(A), w_(B)and w_(C) are used to adjust the relative importance of the variousterms in the merit function according to the requirements of aparticular application. It is recognized that early in the lens designprocess, knowledge of the optical power loads may be limited. Therefore,in some embodiments, the lens design merit function Q can use terms forthermal stress birefringence that express the glass only stressbirefringence metric M₁ instead of the intensity weighted M₂ metric. Thelens design merit function Q can include numerous other terms, includingones that enable automated glass optimization. In this context, otherlens design parameters, such as lens element curvatures (radii),thicknesses, positions (including cemented or not), use of asphericcoefficients, glass choices, or the number of lens elements, are thevariables that are adjusted when attempting to optimize the lensrelative to the merit function Q. These variable parameters can havetargets, ranges, or limits within the lens design merit function.

FIG. 15 shows a flow chart of an exemplary method for designing animaging lens having reduced susceptibility to thermally-induced stressbirefringence which involves optimizing a merit function that includes athermally-induced stress birefringence performance term. An input to thelens design process is a set of lens design requirements 500. The lensdesign requirements would include lens design attributes such as variousgeometrical properties (e.g., focal length, F/#, working distance andmagnification), as well as image quality requirements and thermal stressbirefringence susceptibility requirements.

Another input to the lens design process is a database of glassmaterials data 505 accessible from a memory having glass informationrelevant to the lens design process. Preferably, the glass materialsdata 505 would include refractive indices (n_(d)), Abbe number (ν_(d)),absorption coefficient (α), stress optical coefficient (κ), thermalconductivity (K), coefficient of thermal expansion (ρ), Poisson's ratio(μ) and Young's Modulus (E).

A compute glass-only birefringence metrics step 510 is used to determineglass-only thermal stress birefringence metrics (e.g., M₁) for theglasses in the glass materials data 505 to determine an updated table ofglass materials data with birefringence metrics 515. Generally, glasseshaving a negligible or low susceptibility to thermal stressbirefringence should be used to design imaging lenses having a reducedsusceptibility to thermally-induced stress birefringence. Therefore, itis useful to define two sets of candidate glasses: a first set ofcandidate glasses having a negligible susceptibility to thermal stressbirefringence (e.g., M₁≦0.1×10⁻⁶ W⁻¹), and a second set of candidateglasses having at most a moderate susceptibility to thermal stressbirefringence (e.g., 0.1×10⁻⁶ W⁻¹≦M₁≦1.60×10⁻⁶ W⁻¹).

A determine nominal lens design step 520 is used to determine a nominallens design 525 that stultifies the lens design requirements 500,responsive to the glass materials data 505. Preferably, the determinenominal lens design step 520 is performed using any one of the commonlens design software packages (e.g., Code V™, or Zemax™) available foruse on personal computers leveraging well-known lens design methods.This effort is commonly undertaken using a lens design merit function Qand an iterative optimization process. Generally, such lens designmethods include using the lens design software to determine shapes,sizes, spacings and optical glasses for a plurality of lens elements.The resulting nominal design can be stored in a memory, and used as abaseline for both further optimization and comparison.

A define merit function step 560 is used to define a merit function 565,such as the example given in Eq. 15. The merit function 565 willgenerally include one or more image quality performance terms and athermally-induced stress birefringence performance term in order toselect a lens design that simultaneously produces high image quality andlow stress birefringence susceptibility. It is understood that the meritfunction 565 is mutable, and that a lens designer may add or removeterms from a lens design merit function Q, or change weighting factors,limits, or ranges of acceptability, during the course of the designeffort.

A substitute negligible and low birefringence glasses step 570 is usedto substitute glasses having a negligible or a low susceptibility tothermal stress birefringence for the glasses in the nominal lens design525. This step can be done automatically using software executed by acomputer processor. Alternately, it can be done manually be a lensdesigner. In one embodiment, a lens designer substitutes glasses fromthe first set of candidate glasses for one or more of the lens elementshaving the highest power densities. In many cases, these will be thelens elements located immediately adjacent to the aperture stop. Glassesfor the rest of the lens elements can be selected from either the firstor second sets of candidate glasses. Generally, the glasses that aresubstituted for the original glasses in the nominal lens design 525 aredetermined by identifying the glasses from the first and second sets ofcandidate glasses that most closely match the refractive index anddispersive properties of the original glasses.

Next an optimize lens design step 575 is used to determine an optimizedlens design 580 responsive to the merit function 565. A computerprocessor performs this step using an optimization process to adjustvarious parameters for the lens elements (e.g., thicknesses, spacingsand shapes) to provide the best performance. Both interim and finalresults can be stored in a memory. In some cases, the optimizationprocess can also be used to automatically choose glasses for the lenselements. Since the merit function 565 includes both image qualityperformance terms and a thermally-induced stress birefringenceperformance term, the optimized lens design 580 will balance both ofthese important attributes of the design. The lens designer can chooseto emphasize the importance of one attribute or the other by adjustingthe weights of the corresponding terms in the merit function 565. Thereare many optimization processes well known to those skilled in the artthat can be used to determine the optimized lens design 580. Examples ofcommon optimization processes include damped least squares,orthonormalization and simulated annealing.

In some cases, the lens designer may conclude that the optimized lensdesign 580 determined by the optimize lens design step 575 is notadequate to satisfy the requirements of a particular application. Inthis case, the lens designer may choose to take steps such assubstituting different glasses for one or more of the lens elements,adding additional lens elements, allowing one or more of the surfaces tohave an aspheric surface profile or adding a diffractive opticalsurface. The optimize lens design step 575 can then be executed again todetermine a new optimized lens design 580.

Even without having the benefit of the automated calculation of lensdesign program merit functions that include the thermal stressbirefringence metrics, the lens designer can successfully design lenseshaving a reduced susceptibility to thermal stress birefringence byfollowing the principles of this invention. This can be accomplished byusing the tables of FIG. 8A-8D (or an expanded version thereof) to guidethe lens designer in the selection of optical materials, and then usingthe optimization tools in a conventional lens design program running ona computer to determine an optimized lens design.

Generally, this process will involve defining a set of lens performancecriteria that the imaging lens must satisfy, including one or more imagequality performance criteria, and a thermally-induced stressbirefringence performance criterion. The image quality performancecriteria can, for example, be a specification of the required MTFperformance. The thermally-induced stress birefringence performancecriterion can, for example, be an allowable maximum value for anaggregate thermal stress birefringence metric which combinesintensity-weighted thermal stress birefringence metrics for each of thelens elements.

The most important factor that must be considered in the process ofproducing a lens design that exhibits minimal thermal stressbirefringence, especially under high optical power density conditions,is the selection of the optical glasses. A few glasses are obviouschoices such as Schott SF57 and Ohara PBH56 for their near zero stresscoefficient, and Fused Silica for its extremely low absorption and lowcoefficient of thermal expansion. All other glasses impart some largerdegree of polarization degradation under thermal stress. A figure ofmerit such as the previously discussed glass-only thermal stressbirefringence metric M₁ can be used to create a list that orders theglasses from best (lowest metric value) to worst (highest metric value),such as the list given in FIGS. 8C-8D, or an expanded version thereof.This properties summarized in this glass list are also useful during thelens design process for choosing glass types that have desirable opticalaberration control attributes (index and dispersion). A balance betweenaberration control and thermal stress birefringence control must be madefor the glass choice for each lens element, as was previously discussed.

For illustrative purposes, a lens design process similar to that usedfor designing the third exemplary projection lens 270 of FIG. 12A willnow be described with reference to FIG. 16. Again, in this process, thelens designer has the benefit of a lens design program, a computer witha processor and memory, but a lens design merit function Q that lacksterms for directly optimizing thermally-induced stress birefringence.Instead the lens designer can use the tables of FIGS. 8A-8D, or theirmore complete equivalents, including potentially data from other glassmanufacturers, as input to the lens designer's efforts. As with the FIG.15 process, inputs to the lens design process are a set of lens designrequirements 500 and a database of glass materials data 505 having glassinformation relevant to the lens design process.

As with the FIG. 15 process, a compute glass-only birefringence metricsstep 510 is used to determine glass-only thermal stress birefringencemetrics (e.g., M₁) for the glasses in the glass materials data 505 todetermine an updated table of glass materials data with birefringencemetrics 515. Likewise, a determine nominal lens design step 520 is usedto determine a nominal lens design 525 that stultifies the lens designrequirements 500, but which may include lens elements that use glasseshaving poor thermal stress birefringence susceptibility.

Next a substitute negligible stress birefringence glass step 530 is usedto determine an intermediate lens design 535. Preferably, this stepincludes identifying the lens element in the nominal lens design 525that has an optical glass that most closely matches the refractive indexand dispersion characteristics of a high-index/high-dispersion glasshaving negligible stress birefringence susceptibility (e.g., Schott SF57 or Ohara PBH56). The optical glass for the identified lens element isreplaced by the glass having the negligible stress birefringencesusceptibility glass, and the lens design is then re-optimized using thelens design software.

If there are any other optical elements that have glasses that aresimilar to the high-index/high-dispersion negligible stressbirefringence glasses (e.g., refractive index within ±0.1 and ν-numberwithin ±20) then the substitute negligible stress birefringence glassstep 530 can be applied iteratively until the optical glasses for allsuch lens elements have been substituted.

Similarly, if there are any other optical elements that have glassesthat are similar to a low-index/low-dispersion negligible stressbirefringence glass (e.g., fused silica) then the substitute negligiblestress birefringence glass step 530 can be applied iteratively tosubstitute the optical glasses for all such lens elements with thelow-index/low-dispersion negligible stress birefringence glass andre-optimize the lens design using the lens design software.

In another embodiment, the substitute negligible stress birefringenceglass step 530 identifies which elements in the nominal lens design 525have the highest optical power densities and substitutes negligiblestress birefringence glass (e.g., a glass where M₁≦0.1×10⁻⁶ W⁻¹) forthose lens elements. Generally, the negligible stress birefringenceglass that most closely matches the refractive index and ν-number of theoriginal optical glass of the identified lens element is used tosubstitute for the original optical glass. Generally, the lens elementshaving the highest power densities will be the lens elements that areclosest to the aperture stop 230. Therefore, it is not always necessaryto compute the optical power densities. In some embodiments, the opticalglass for the lens elements immediately adjacent to the aperture stopare automatically substituted using negligible stress birefringenceglass.

Next, a substitute low stress birefringence glass step 540 is used todetermine a new intermediate lens design 545 by substituting the opticalmaterial for one of the lens elements with an optical material having alow stress birefringence susceptibility (e.g., M₁≦0.8×10⁻⁶ W⁻¹).Preferably, this step includes identifying the low stress birefringencesusceptibility glass from the glass materials data with birefringencemetrics 515 that most closely matches the refractive index and ν-numberof a selected lens element. The optical glass for the selected lenselement is replaced by the low stress birefringence susceptibilityglass, and the lens design is then re-optimized using the lens designsoftware. The substitute low stress birefringence glass step 540 can beapplied iteratively until low stress birefringence susceptibilityglasses have been substituted for all of the lens elements.

Next an evaluate composite metrics and update lens design step 550 isused to determine a final lens design 555. For this step, theintermediate lens design 545 is analyzed to determine whether it hasacceptable image quality performance. It is also analyzed to determinethermal stress birefringence metrics for the lens elements thatincorporates the lens element geometry (e.g., M₂), provided thatestimates of the optical flux loading on the different lens elements isavailable. If the image quality performance of the intermediate lensdesign 545 has been degraded too much, than alternate glass choices canbe made. For example, the optical glass for those lens elements whichexperience a lower power density can be updated to select materials froman expanded set of optical glasses where the threshold thermal stressbirefringence metrics is increased somewhat to include intermediatestress birefringence susceptibility glasses (e.g., M₁≦1.6×10⁻⁶ W⁻¹)

An additional supportive step is possible, in which the lens design ismodeled thermal-mechanically, for example by finite element analysis(FEA), to model the light absorption volumetric profiles within the lenselements, the resulting heat load and temperature changes ΔT, and thenthe resulting induced stress birefringence Δn. This modeling can accountfor heat conduction, convection, and radiation effects within or aroundthe lens elements and the housing 240, including any lens coolingtechniques applied to reduce the thermal effects imparted by lightabsorption. The results can then be used to test or validate the lensdesign expectations, including the use of the M₂ metric. The resultinginformation can guide the lens design process, verifying the need fornegligible, low, or moderate thermal stress birefringent glasses forgiven lens elements, or indicating opportunities to select lessrestrictive glasses.

In some cases, it may also be necessary to relax one or more of theglass selections made in the substitute negligible stress birefringenceglass step to use a low stress birefringent glass instead of theoriginal negligible stress birefringent glass. If so, all of the glassesthat were selected by the substitute low stress birefringence glass step540 should be reevaluated.

If the resulting thermal stress birefringence susceptibility is toohigh, or if the image quality performance is too low, then lens elementscan be selectively chosen where the use of aspheric surfaces can provideincreased image quality performance while allowing glass materials to beused having a tighter threshold thermal stress birefringence metric(e.g., M₁≦0.8×10⁻⁶ W⁻¹). Emphasis can be directed to changing glassesfor lens elements that experience the highest optical power densities.

While the exemplary lens designs given in the description of the presentinvention have been based on a classical double Gauss lens design, itwill be obvious to one skilled in the art that the same basic designprinciples can be used in the design of any other type of imaging lens,or systems of imaging lenses, where there is a need to reducesusceptibility to thermally-induced stress birefringence. For example,the design principles could be used to design imaging lenses fallinginto well-known lens design categories such as anastigmat lenses,Petzval lenses, telephoto lenses, zoom lenses, afocal lenses, andF-theta lenses.

As a particular example, F-theta lenses are used in laser printing inpost objective or pre-objective scanner configurations, where a scannersuch as a galvanometer is located proximate to the aperture stop plane,and all the lens elements of the F-theta objective lens are located onone side (pre- or post-) of the aperture stop plane. F-theta lensestypically have all their lens elements clustered in one group, to eitherside of the aperture stop. Again, the lens elements nearest the aperturestop are the elements most likely to benefit from careful glass choiceselections for reducing thermally-induced stress birefringence. Asanother example, zoom lenses can have multiple lens groups (2-4 forexample), including moving groups.

The lens design concepts of the present invention are not limited inapplication to imaging lenses, but can be used for other lens systemssuch as illumination systems, or in combined reflective-refractiveoptical systems such as those based on telescope type designconfigurations. Again, the lens elements proximate to an aperture stop,or other elements experiencing high power densities are the elementsmost likely to benefit from the use of reduced stress birefringencesusceptibility glasses. In accordance with the present invention, lenssystems of any type can be made by selecting negligible stressbirefringent susceptibility glasses for use with the lens elementshaving the highest optical power densities, and then using glasseshaving at most a moderate stress birefringent susceptibility for theremaining lens elements.

It is noted that the image quality of the exemplary projection lenses270, per FIGS. 6B and 9B exhibit a limited MTF performance (˜30%) at 100cy/mm. To support the upper-end digital cinema industry specificationfor on-screen resolution, the ability to project discernable pixels with4K horizontal resolution is required. To better achieve this goal, itwould be preferable that the MTF at 100 cy/mm be improved to exceed 40%.Likewise, it would also be preferred to provide better MTF performanceat 50 cy/mm (˜75% or better, instead of 60-65%.) to provide improvedimage quality for 2K resolution images. At the same time, it would alsobe preferable to further reduce the susceptibility of the relay lens 250and projection lens 270 to thermally-induced stress birefringence, toprovide more margin, as well as to better enable brighter screens (e.g.,20,000-60,000 lumens).

There are several potential approaches to achieving these goals, whichcan be applied either individually or in combination. Certainly, asdiscussed previously, strategic choices of select alternate glasses,such as LF-5 or S-NSL36, can help some lens elements significantly. Theuse of aspheric surfaces on one or more lens elements can also be veryhelpful. As another approach, the optical speed of the system can beincreased, such that the projection lens 270 operates at F/2.5 orfaster, so as to enhance the MTF performance. Although the projectionlenses 270 are aberration limited, rather than diffraction limited, thiscan still provide some improvement. Increasing the system speed can alsohelp reduce the thermally-induced stress birefringence because the lightwill be spread over more glass area, thereby reducing the peak powerdensities. However, this approach can make the individual lens elements,as well as the lens assembly, more expensive and more difficult todesign and fabricate.

It is also recognized that the lens housing 240, or more directly thelens elements themselves, can be cooled, either passively or actively,using heat sinks, conductive tapes, liquid cooling, passive or forcedair, thermal-electric cooling devices, or other techniques, with thegoal of reducing the effective thermal load on the lens elements, andthus the induced stress birefringence. Such cooling, if it issufficiently effective and reliable, can change the glass choices forone or more lens elements. It is also recognized that the mounting oflens elements within the lens housing 240 needs to be done judiciously,so that thermal expansion of the lens elements against internal housingfeatures does not cause stress birefringence from induced mechanicalstress.

In considering image quality issues, it is noted that the MTFperformance of the projection lens 270, including that of the thirdexemplary projection lens (FIG. 12A) is limited by the opticalaberration known as primary axial color or longitudinal color. In a lensdesigned with primary axial color correction, the best focus plane forgreen light is offset from the best focus blue and red planes. Thatmeans that red and blue channels have planes of best focus that overlapat the same place in image space (at or near display surface 190), butthey are both longitudinally shifted from the green channel's plane ofbest focus.

If the normal lens design rationale were followed, improved colorcorrection can be obtained by designing the projection lens 270 as anapochromat, much as was done in the previously cited U.S. Pat. No.5,172,275 to D. DeJager. Typically in lens design, secondary axial coloris eliminated by designing lens assemblies with one or more lenselements consisting of an anomalous dispersion glasses such as theSchott PSK and NKZFS glass types. Unfortunately, these glasses haveamong the worst values for thermal stress birefringence susceptibility(M₁18 3.5×10⁻⁶ W⁻¹), and thus trying to fix axial color with theseglasses will significantly increase the stress birefringence at the sametime. Schott PK52 and Schott FK51 also provide useful partialdispersions, and are nearly 10× better for thermal stress sensitivity(M₁<8 0.36×10⁻⁶ W⁻¹), but their locations on the glass chart (lowindices (n_(d)) and high Abbe numbers (ν_(d)) bring other compromises.

Alternately, as discussed in commonly assigned U.S. Pat. No. 6,317,268to M. Harrigan, entitled “Movie Projection Lens,” the inclusion of adiffractive optical element (DOE) is particularly useful for correctingcolor aberrations, including axial color (axial chromatic aberration).Diffractive optical elements are patterned with step structures thatform thin phase elements and operate by means of interference anddiffraction to effect light propagation. They can be formed on planarelements or curved lens surfaces. In the case of the present invention,one or more diffractive optical elements 350 can be provided within aprojection lens 270 or the relay lens 250, such as at the exemplarypositions 90 depicted in FIG. 6A. The use of diffractive opticalelements can improve the image quality, and can also enable alternateglass selections, making it easier for the lens designer to use glasseswith lower stress birefringence susceptibility (such as a weak-crownlike Ohara SNSL36 or a weak-flint like Schott LF5) in key locations.However, diffractive optical elements can introduce flare light, therebycausing degradation in ANSI or in-frame contrast, so care is required.

Further consideration of the second exemplary projection lens 270 ofFIG. 10A, which comprises only the very low stress birefringent PBH56and fused silica glasses, together with the corresponding table in FIG.11A, indicates that there is considerable room available to furtherreduce susceptibility to thermally-induced stress birefringence if glassselections and design performance would allow it. As another approach,adding a polarization compensator 360 within projection lens 270, forexample at position 90 of FIG. 4, can also be beneficial. In priordiscussions, specifically with regard to the related patent by Aastuenet al. and FIG. 1B, the use of polarization compensators 360 wasdiscussed in part. Within projector 101, polarization compensatorstypically reside within the modulation optical system 80, and providepolarization compensation to the spatial light modulators 170 r, 170 gand 170 b, polarization beamsplitters 60, or both in combination. Forexample, in U.S. Pat. No. 6,909,473 to Mi et al., polarizationcompensation can be provided for both a vertically aligned LCD typemodulator and a wire grid polarization beamsplitter. In the case ofAastuen et al., similar polarization compensation expands the list ofcandidate glasses for the substrate glass of a thin film,glass-embedded, polarization beamsplitter.

There are numerous examples of polarization compensators developed toenhance the polarization performance with LCDs, including those designedfor vertically aligned or nematic LCDs. These compensators typically usepolymer films to provide angular varying birefringence, structured in aspatially variant fashion, to affect polarization states in portions(that is, within certain spatial and angular areas) of the transitinglight beam, without affecting the polarization states in other portionsof the light beam. Alternately, as described in U.S. Pat. No. 7,170,574to Tan et al., robust polarization compensators have been developed bycreating form birefringent grating structures with dielectric thin filmlayers deposited on glass substrates. In general, the polarizationcompensators counteract the differential effects experienced by thepolarization states of skew rays or oblique rays, which traversedifferent optical paths through birefringent materials, than do morenormally incident rays.

In the instance of the present invention, the polarization compensator360 does not reside in a modulation optical system 80, to adjust thepolarization states of the traversing light beams 140 relative to aspatial light modulator 170 and a polarization beamsplitter 60. Instead,the polarization compensator can be provided at a position 90 withinprojection lens 270, and can provide polarization compensation forstress birefringence effects within the projection lens. In particular,and it can provide compensation for residual thermally-induced stressbirefringence. In this case, the compensator restores the polarizationstates or orientations that deviate from their incident conditions inaccordance with the stress birefringence, such that they better resemblethe incident states, and then provide the desired left-eye or right-eyepolarization states to the viewers, who are wearing polarizationsensitive glasses. The incident polarization states were, for example,largely defined in the illumination assembly 110, by the laser devicesand the polarization switch 139. It should be understood that one ormore polarization compensators 360 can also be provided within or aboutrelay lens 250 to compensate for stress birefringence effects caused bythat lens.

As shown in FIG. 4, polarization compensators 360 can be located atvarious exemplary positions 90 within the imaging lens 200, or at aposition 90 proximate to both the dichroic combiner 165 and the relaylens 250. Polarization compensation can correct for polarization effectscaused by multiple components, including the projection lens 270, therelay lens 250, and the dichroic combiner 165. Accordingly, the use ofpolarization compensators 360 can enable the projection lens 270 orrelay lens 250 to be designed with a wider range of glasses thanotherwise, as the effects of thermally-induced stress birefringence areotherwise reduced. That can provide design freedom to further improveimage quality.

Returning to the problematic issue of residual secondary axial coloraffecting the third exemplary projection lens (FIG. 12A), the red andblue channels have planes of best focus that overlap at the same placein image space (at or near display surface 190), but that are bothlongitudinally shifted from the green channel's plane of best focus. Asthe spatial light modulators 170 reside in different planes in FIG. 1,this effect can be reduced by adjusting the modulator positions, butwith a resulting penalty of a small magnification difference. The colorfocus error is large enough that it can be noticed by a criticalobserver, and this problem can be exacerbated at higher resolutions. Asanother alternative to using polarization compensators 360 ordiffractive optical elements 350, an additional optical element thatincreases the path of the red and blue light with respect to the greenlight can be used. This will effectively place all three colors at acommon focus at the image plane. A simple way to accomplish this makesuse of a first surface mirror 370, as shown in FIG. 17, where greenlight is reflected from the top layers of a multilayer coating 375 whiletransmitting the blue and red light. The red and blue light is thenreflected from layers that are deeper into the multilayer dielectriccoating 375 such that the increased travel distance to this lower layeris the same as the amount of secondary axial color in the projectionlens 270.

First surface mirror 370 is shown in exaggerated fashion, as the amountof secondary axial color in the projection lens 270 is about 50 micronsin object space, so the depth of the coating layers between the top andbottom will be less than that because the red and blue light experiencetwo passes through the material indices of the coating layers (reducedOPD). The green and red/blue images are also shifted laterally from oneanother by a small image shift 378 at the intermediate image 260, butthis can be adjusted by repositioning the spatial light modulators 170appropriately.

It is noted that this concept can be extended to correct for primaryaxial color also, by having three reflective layers formed on thesubstrate 377, one for each color. This approach can be used to correctfor the relay lens 250, the projection lens 270, or both in combination.Again, the thicknesses would be adjusted to compensate for thelongitudinal shifts of the colors.

The first surface mirror 370 is shown at 45 degrees in FIG. 16, but theidea works for correcting axial chromatic aberrations with any tiltangle as long as the light paths are not blocked by any physicalobstructions. Additionally, the first surface mirror 370 can beangularly adjusted to change the amount of color correction introduced.If image shift is a problem, a two reflection arrangement can be used tocounteract the shift, for example by positioning two reflectivesurfaces, each with a corrective coating, in a penta prism typeconfiguration.

The present invention provides a basis for understandingthermally-induced stress birefringence, including numeric metrics M₁ andM₂ (the latter accounting for the applied optical loading), anidentification of glasses having comparatively negligible, low,moderate, or high values of thermal stress susceptibility (as measuredby M₁), and supporting design principles and methods for selectivelyusing these glasses for different lens elements within lens assembliesduring lens design efforts. The present invention has also providedpractical design examples of imaging lenses having differing levels ofthermal stress susceptibility and imaging performance, and practicalevidence of their impact on improved projector performance. Inparticular, a set of thermal stress susceptibility thresholds has beenidentified, which are used to designate negligible stress glasses(0.1×10⁻⁶ W⁻¹≦M₁), low stress glasses (0.1×10⁻⁶ W⁻¹≦M₁≦0.8×10⁻⁶ W⁻¹),and moderate stress glasses (0.8×10⁻⁶ W⁻¹≦M₁≦1.6×10⁻⁶ W⁻¹), each ofwhich include a set of glasses with valuable properties for lens design.It should be understood that within the spirit and scope of the presentinvention, that these thresholds are somewhat arbitrary. For example, aglass (perhaps not yet developed) having a value of M₁=0.12×10⁻⁶ W⁻¹ maybe considered to have either a negligible or low thermal stressbirefringence sensitivity. Indeed, the perception thereof may depend onthe application or the lens designers involved. Likewise, the otherthresholds, for low M₁≦0.8×10⁻⁶ W⁻¹), and moderate (M₁≦1.6×10⁻⁶ W⁻¹)thermal stress birefringence glasses are useful and effective, but againare somewhat arbitrary. For example, the low threshold could be set atM₁≦0.9×10⁻⁶ W⁻¹ or M₁≦1.0×10⁻⁶ W⁻¹, and the moderate threshold could beset at M₁≦1.8×10⁻⁶ W⁻¹.

In the prior discussions, an approach to designing imaging optics withreduced susceptibility to thermal stress birefringence was presented. Assuch imaging optics are required to handle increased flux, for examplewithin a projector illuminating a yet larger display screen 190, stressbirefringence and polarization contrast loss can re-emerge. As oneapproach, imaging optics, whether a relay lens or projection lens, canbe designed with only negligible stress glasses (fused silica and SF-57)but with a multitude of aspheric surfaces. However, cost increases asmore aspheric surfaces are added, and the increasing number of asphericsurfaces provides diminishing returns relative to improving imagequality. By comparison, judicious choices of low-to-moderate thermalstress birefringent crown and/or flint glasses from the tables of FIGS.8A-8D can provide a significant improvement in image quality. Thus,while use of a plurality of aspheric surfaces can reduce the number ofelements having low or moderate thermal stress birefringent glasses, andthus reduce the susceptibility of the lens assembly to thermal stressbirefringence in aggregate, selective use of a wider range of glassesdoes provide significant value.

As another approach, it was previously stated that low susceptibility tothermal stress birefringence imaging optics can include combinedreflective-refractive optical systems such as those based on telescopetype design configurations. In particular, this means that the portionsof the imaging optics having optical power can be refractive, catoptric(all reflective) or catadioptric (both reflective and refractive). (Flatelements, such as windows or optical filters are secondary to thisdiscussion.)

FIG. 18 depicts one particular example of a projector 102 a whichincludes catadioptric imaging system. This configuration is similar tothe projector 102 shown in FIG. 2 where the imaging lens 200 has beenreplaced with a two-stage imaging system including an Offner relay 290and a projection lens 270 to project the image formed using the spatiallight modulators 170 onto the display surface 190. The Offner relay 290is a one-to-one object-to-image relay system using two concentricmirrors, a primary mirror 295 and a secondary mirror 292, to provide theintermediate image 260 as input to the projection lens 270.

The Offner relay 290, which was originally described in U.S. Pat. No.3,748,015 entitled “Unit power imaging catoptric anastigmat,” by A.Offner of Perkin Elmer, is a catoptric (all reflective) system that wasdeveloped for the Micralign microlithography system and used to image a6″ diameter mask at unity magnification onto a 6″ diameter siliconwafer. The primary mirror 295 is concave, while the secondary mirror 292is convex, with the two front surface mirrors having a concentricrelationship about a common point on their local optical axis.

The Offner relay 290 system is afocal (doubly telecentric), with itsentrance pupil at infinity. The aperture stop 230 of this optical systemis located at the secondary mirror 292. This system is corrected for allthird order aberrations and for a number of higher order aberrations.Also, as this system relies on the two spherical mirrors (primary mirror295 and secondary mirror 292) to create a real image, without requiringrefractive elements (glass or polymer), index dispersion does not affectimaging, and there are no chromatic aberrations. As a result, theimaging performance, such as MTF is superior.

As shown in the FIG. 18 configuration, two fold mirrors 160 are used,one to collect light from the object (spatial light modulators 170), andthe other to direct light toward the projection lens. The primary mirror295 and the secondary mirror 292 share a common optical axis 145′, forwhich aperture 230 is on-axis, but the field (object and image) are“off-axis.” The design performance can then be optimized over an annularregion between a minimum and a maximum radial field. That is, with theconcentric layout and folds from mirrors 160, the area of best imagequality at the image plane is annular, and centered at the optical axis145. An example of an imaging system which benefits from the superiorimage quality available in an arcuate field is provided in commonlyassigned U.S. Pat. No. 6,304,315 entitled “High speed high resolutioncontinuous optical film printer for duplication motion films”, byKessler et al. That said, the Offner relay 290 can still be used toeffectively image a two dimensional or rectangular area at intermediateimage 260, rather than just an annular area. In the meridional plane,the field is limited by high order astigmatism and high order sphericalaberration. Offner relays 290 can also vary modestly from unitymagnification, such as to provide the preferred near unity magnification(˜1.04×) for the relay imaging of the present projector, with minimaldegradation in the imaging performance.

In the case of the embodiment of the projector 102 a depicted in FIG.18, replacing the relay lens 250 (FIG. 4) and its lens elements 205(FIG. 4) with the Offner relay 290 and its mirrors (primary mirror 295and secondary mirror 292) not only provides superior image quality buteliminates several lens elements that can otherwise be subject tothermally-induced stress birefringence. The projection lens 270 ispreferably designed for reduced susceptibility to thermally-inducedstress birefringence, in accordance with the methods of the presentinvention, so that the projector 102 a has an overall reducedsusceptibility. Moreover, some of the enhanced image quality provided bythe Offner relay 290 can be sacrificed to provide greater latitude inglass selection during the design of the projection lens 270. Inconsidering the projector 102 a of FIG. 18, compared to the projector ofFIG. 2 and the imaging lens 200 of FIG. 4, it becomes apparent that onedownside of using the Offner relay 290 is the increased volume occupiedby the mirrors (primary mirror 295 and secondary mirror 292) and thebeam paths. While the drawing of FIG. 18 is not to scale, it isnonetheless true that an Offner relay 290 requires a lot of space.

Alternately, the Offner relay 290 can be modified to be a hybrid orcatadioptric system, comprising both refractive and reflective elements.A related exemplary catadioptric system is described in commonlyassigned U.S. Pat. No. 6,014,272 entitled “Retroreflective lens”, byArnold, which can reduce the space required by a traditional Offnersystem at the cost of reintroducing more glass elements, and thereforemore potential for thermally-induced stress birefringence. The designapproach of Arnold ('272) provides a lens group including a convex lenswith a mirrored surface, with the reflective surface provided on theback side of a refractive element. This approach can be modified tofollow the present inventive method, in which a lens group uses fusedsilica, SF-57, or equivalent optical materials with negligible thermalstress birefringence susceptibility in combination with other glasses oroptical materials having only low or moderate thermal stressbirefringence susceptibility, as exemplified in the tables of FIGS.8A-8D.

For example, in considering FIG. 18, refractive elements can bepreferentially placed near the primary mirror 295, rather than thesecondary mirror 292, as the incident optical flux densities arereduced. The primary mirror 295 can also be a Mangin mirror, with amirrored surface on the rear surface of a concave lens, where glass isjudiciously chosen as negligible-to-moderate thermal stress birefringentsusceptible glasses. Likewise, lens elements can be near the secondarymirror 292, or the secondary mirror 292 can comprise a glass elementwith a mirrored rear surface, again with glass(es) that are selected tohave negligible-to-moderate induced thermal stress birefringence. As afurther point of design freedom, an Offner relay 290 or an Arnold ('272)type relay can provide an intermediate image 260 with non-unitymagnification (for example 1.5× or 2×). In such cases, additionalmirrors, such as a third mirror having optical power and acting as acorrector, can be needed near the object plane.

Alternately, it should be understood that the relay optics can remain asa refractive relay lens 250 (FIG. 4) comprising refractive (glass orpolymer) lens elements 205, and catoptric or catadioptric designs usingreflective elements can be used for the projection optics. In the casethat the projection optics are catoptric (all reflective), they caninclude telescopic optics configurations. Most classical telescopicforms, such as Cassegrain or Ritchey-Chretien designs, are used forastronomy and have a narrow field of view and an infinite conjugate.However, there are telescopic imaging optics that provide finite (thoughdistant) conjugate imaging and large fields of view. Also, manyclassical telescopic design forms have a central obscuration, whichvignettes incoming or outgoing light. Alternate design forms withoff-axis mirrors can avoid the central obscuration problem, althoughdesigning such systems to avoid clipping or vignetting with the beampassing nearby mirror surfaces can be difficult.

Off-axis telescopic designs often have three primary mirror elements andtake the form of a “reflective triplet” that lacks an intermediateimage. Alternately, they may take the form of a “three-mirroranistigmat” that generally includes an intermediate image. Typically,the mirrors are a combination of surfaces with elliptical, hyperbolic,or parabolic profiles. These designs can provide telescopic imaging withhigh image quality, while avoiding the limitation of a centralobscuration. While the reflective triplet and three-mirror anistigmatapproaches are often called “three mirror systems,” that is typicallythe minimum number, and more mirrors can be used in some designs.Alternately, or in addition, some or all of the mirrors can be free-formor freely shaped, using complex aspheric designs, as is sometimes donefor short throw projection. For example, U.S. Patent ApplicationPublication 2006/0227303, entitled “Projection Display Apparatus” byMatsubara et al., describes a short throw projector that uses off-axismirrors having free form profiles in the design.

While off-axis telescopic designs avoid the central obscuration problem,there are fabrication issues. For example, when a design requires onlyan off-axis portion of a surface of rotation (aspheric or not),significant expenses can be incurred from scrapping the unused portionsof a mirror surface, or in developing molds to provide replicableoff-axis mirror portions. Additionally, telescopic designs, much likethe Offner relay, tend to occupy a substantial amount of space, whichcan cause design conflicts in integrating the projection optics with therest of the projector. For such reasons, catadioptric telescopicprojection optics can represent a balanced approach for reducingthermally-induced stress birefringence while easing mirror design andfabrication issues.

Catadioptric projection optics (having both refractive and reflectiveelements) have previously been used for short throw projectors, asexemplified by U.S. Pat. No. 7,163,297, entitled “Image display system”,by Suzuki et al. However, the design of catadioptric optics in which theglass or optical materials of the lens elements therein are alsoselected to minimize the occurrence of thermally-induced stressbirefringence has not been previously disclosed.

As an example, FIG. 19 depicts catadioptric projection optics 275 whichcan be used for an alternate embodiment of the present invention inwhich the catadioptric projection optics 275 are used in place of theprojection lens 270 of FIG. 4. The catadioptric projection optics 275includes both a group of lens elements 208 and two projection mirrors280 and 285. In this example, which can be used in projector 102 (FIG.2), several of the lens elements beyond the aperture stop 230 have beenreplaced by projection mirrors 280 and 285. While the lens elementswithin group of lens elements 208 would be designed with considerationfor both aberration corrections of the projection optics and reducingsensitivity to thermally-induced stress birefringence, the overallfigure of merit for the lens would be roughly halved as half the lenselements have been replaced.

While reflective telescopic projection optics benefit from a lack ofchromatic aberrations, concerns can arise relative to controllingaberrations such as distortion or field curvature over large fields ofview while operating with fast F/#'s. The use of aspheric or free-formmirror surfaces can help resolve these problems, but mirror surfacesthat are not surfaces of revolution are more expensive to fabricate. Theuse of refractive optics can provide more aberration control and help toenhance the imaged field, but at the cost of introducing chromaticaberrations.

As depicted in FIG. 19, the catadioptric projection optics 275 includesprojection mirrors 280 and 285, together with five refractive lenselements in the group of lens elements 208 arranged about the aperturestop 230. This system can be considered as a variant of a reflectivetriplet system, with the group of lens elements 208 providing thefunctionality of a first mirror.

FIGS. 20A-20B provide a table specifying the lens design parameters forthe catadioptric projection optics 275 of FIG. 10. Within the group oflens elements 208, the two lens elements about the aperture stop 230,lens elements 460 (element #7 in FIG. 20A) and lens element 461 (element#6 in FIG. 20B), are fabricated using the negligible stress birefringentglasses SF-57 and fused silica, respectively. Lens element 460 has anaspheric surface profile, which can be fabricated into the lens shapewhere the entirety of element 460 is glass. Alternately, lens element460 can provide the basic lens shape and curvature, and a thin polymerfilm can be added to provide the aspheric surface corrections. The nextthree lens elements of group of lens elements 208 are fabricated usinglow thermal stress birefringent glasses (SF-4 and SLAL-18) selected fromthe tables of FIGS. 8A-8D. Projection mirrors 280 and 281 are bothoff-axis (i.e., de-centered) mirror portions having aspheric surfaceprofiles.

It should be understood that other combinations or designs ofcatadioptric projection optical systems are possible besides the exampleshown in FIG. 19. For example, reflective elements can be used near theaperture stop 230 and refractive elements can be used away from thestop, such as near the intermediate image plane 260. This would have theadvantage of shifting the glass elements away from the aperture stop230, where the highest optical flux densities typically occur, and thusreducing the intensity scaling of thermally-induced stressbirefringence. The refractive elements that remain could be selectedfrom the negligible or low-to-moderate thermal stress birefringenceglasses (or polymers or other optical materials) listed in the tables ofFIGS. 8A-8D, or their equivalents. However, as the discussion relativeto the tables in FIGS. 13A and 13B makes clear, lenses located near theaperture stop 230 that are designed with negligible stress glasses(e.g., fused silica or SF-57) can provide very low thermal stressbirefringence contributions, despite the relatively high powerdensities. Whereas, lens elements away from the stop 230, which cancontribute significantly to aberration correction, may provide thelargest contributions to the thermally-induced stress birefringence,even with the lower applied flux densities. This is because the thermalstress birefringence susceptibility of the low stress glasses is not aslow as would be desired. Therefore, as another alternative, thecatadioptric projection optics 275 can be designed to preferentiallyhave negligible thermal stress birefringence glasses near the aperturestop 230 (on one or both sides thereof) and projection mirrors 280 or285 or equivalents away from the stop, on one or both sides (upstreamtowards the intermediate image 260; or downstream towards the distantdisplay surface 190).

Of course, when refractive elements are used in an optical system, coloraberrations are introduced due to the dispersion of the materials. Twodifferent types of chromatic aberrations may need to be corrected toachieve the highest possible performance.

The first type of chromatic aberration, known as axial color, is thelongitudinal shift in focus of the optical system as the wavelengthchanges. This can be corrected with careful choice of materials thathave different magnitudes of dispersion being used in elements ofdiffering optical powers. That is why it is so important that lensdesigns, such as those exemplified in the tables of FIGS. 13A-13B,include glasses with both high and low dispersion (i.e., flint and crownglasses). For correction of axial color, the position of the lenselements relative to the aperture stop is not a significant factor.

The second type of chromatic aberration, known as lateral color, is thevariation of image size as the wavelength changes. For narrowfield-of-view imaging systems, this may not be a significant issue. Asthe field-of-view increases, this error becomes more dominant. Iflateral color needs to be corrected, it is most effectively dealt withby placing refractive components on both sides of the aperture stop. Thesign of lateral color will change for elements on one side of theaperture stop 230 versus the other. So, for wider field of view systems,it would be preferred to surround the aperture stop 230 with refractiveelements to control lateral color selected from the glass types with thelowest thermal stress birefringence susceptibility. In this context,FIG. 19 gives an example of a wide field-of-view (e.g., 30° full field)projection lens design having refractive elements located on both sidesof the aperture stop 230 to help correct lateral color. For comparison,FIG. 21 depicts narrow field of view catadioptric projection optics 275a in which reflective elements (projection mirrors 280 a and 285 a) areused only on one side of the aperture stop 230, while refractiveelements (group of lens elements 208) are used on the other side of theaperture stop 230.

It should be understood that the imaging optics of the present inventioncan also include flat reflective surfaces or fold mirrors that do nothave optical power that can be positioned at various locations in theimaging path. Such surfaces are generally used to redirect the opticalpath. As another point, while the discussion has focused on the use ofoptical glasses, optical materials, including polymers and compositematerials, can be used for optical elements within the imaging optics ofthe present invention. However, in general optically transparentpolymers, such as polycarbonate or polystyrene, are more birefringentand thermally variable than are glass materials. For example,polycarbonate has a thermally induced stress birefringencematerials-only figure of merit M₁˜1000 W⁻¹. By comparison, materialslike PMMA or cyclic olefin copolymers, such as Zeonex from ZeonCorporation (Lousiville, Ky.) are known as optical polymers havingreduced birefringence, and yet their thermally induced stressbirefringence materials-only figure of merit M₁˜80-100 W⁻¹, which is˜10⁵× greater than fused silica. Nonetheless, other optical materials,such as nanocomposite materials (e.g. glass particulates are imbedded inpolymer) or the birefringence-free acrylics (e.g. optical resin OptorezOZ-1330 developed by Hitachi chemical) or transparent ceramics (e.g.tetragonal or cubic zirconia (ZrO2) ceramics) may become useful inoptical elements in accordance with the present invention.

It should also be understood that the imaging optics of the presentinvention can have both relay lens and projection lens equivalents thatare simultaneously catadioptric and used in tandem within the laserprojector. In this case, the refractive elements (glass, polymer, orother optical materials) would preferentially be designed following theglass selection options given in the tables of FIGS. 8A-8D, or theirequivalents, with consideration for the optical densities applied to thelens elements.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the scope of theinvention. It is emphasized that the apparatus or methods describedherein can be embodied in a number of different types of systems, usinga wide variety of types of supporting hardware and software. It shouldalso be noted that drawings are not drawn to scale, but are illustrativeof key components and principles used in these embodiments.

PARTS LIST

-   50 light source-   55 prism assembly-   60 polarization beamsplitter-   65 X-prism-   80 modulation optical system-   90 position-   100 projector-   101 projector-   102 projector-   102 a projector-   110 illumination assembly-   110 r illumination assembly-   110 g illumination assembly-   100 b illumination assembly-   115 light source assembly-   120 laser array-   120′ laser array-   122 laser emitter-   125 laser combining assembly-   127 faceted prism-   130 aperture-   135 illumination beam combiner-   137 half wave plate-   139 polarization switch-   140 light beam-   140′ individual light beam-   145 optical axis-   145′ optical axis-   150 illumination lens-   155 light integrator-   160 mirror-   165 dichroic combiner-   166 first combiner-   167 second combiner-   170 spatial light modulator-   170 r spatial light modulator-   170 g spatial light modulator-   170 b spatial light modulator-   180 despeckler-   190 display surface-   195 object surface-   200 imaging lens-   205 lens element-   206 a positive lens element-   206 b negative lens element-   208 group of lens elements-   210 glass chart-   215 crown glasses-   217 flint glasses-   220 crown lens element-   222 flint lens element-   230 aperture stop-   235 optical rays-   240 lens housing-   245 working distance-   250 relay lens-   260 intermediate image-   270 projection lens-   275 catadioptric projection optics-   275 a catadioptric projection optics-   280 projection mirror-   280 a projection mirror-   285 projection mirror-   285 a projection mirror-   290 Offner relay-   292 secondary mirror-   295 primary mirror-   300 modulation transfer function (MTF) plot-   320 light intensity distribution-   321 slice positions-   322 cross-sectional profiles-   323 light intensity distribution-   324 slice positions-   325 cross-sectional profiles-   326 light intensity distribution-   327 light intensity distribution-   328 light intensity distribution-   350 diffractive optical element-   360 polarization compensator-   370 first surface mirror-   375 multilayer coating-   377 substrate-   378 image shift-   400 lens element-   401 lens element-   402 lens element-   403 lens element-   404 lens element-   405 lens element-   410 lens element-   411 lens element-   412 lens element-   413 lens element-   414 lens element-   415 lens element-   416 lens element-   420 lens element-   421 lens element-   422 lens element-   423 lens element-   424 lens element-   425 lens element-   426 lens element-   427 lens element-   428 fused silica lens element-   429 PBH56 lens element-   430 lens element-   431 lens element-   432 lens element-   433 lens element-   434 lens element-   435 lens element-   436 lens element-   437 fused silica lens element-   438 SF-57 lens element-   440 lens element-   441 lens element-   442 lens element-   443 lens element-   444 lens element-   445 lens element-   446 fused silica lens element-   447 PBH56 lens element-   448 S-LAL18 lens element-   450 lens element-   451 lens element-   452 lens element-   453 lens element-   454 lens element-   455 lens element-   456 lens element-   457 PBH56 lens element-   458 S-LAL18 lens element-   459 fused silica lens element-   460 lens element-   461 lens element-   500 lens design requirements-   505 glass materials data-   510 compute glass-only birefringence metrics step-   515 glass materials data with birefringence metrics-   520 determine nominal lens design step-   525 nominal lens design-   530 substitute negligible stress birefringence glass step-   535 intermediate lens design-   540 substitute low stress birefringence glass step-   545 intermediate lens design-   550 evaluate composite metrics and update lens design step-   555 final lens design-   560 define merit function step-   565 merit function-   570 substitute negligible & low birefringence glasses step-   575 optimize lens design step-   580 optimized lens design

1. An imaging system having reduced susceptibility to thermally-inducedstress birefringence, for projecting an image of an object plane onto adisplay surface, comprising: imaging optics including relay optics thatimage the object plane onto an intermediate image plane and projectionoptics that image the intermediate image plane onto the display surface;wherein one of either the relay optics or the projection optics is areflective optical system that includes reflective optical elements, andthe other of the relay optics or the projection optics is a refractiveoptical system having a negligible or low susceptibility to thermalstress birefringence, the refractive optical system including: a firstgroup of refractive lens elements located upstream from an aperturestop; and a second group of refractive lens elements located downstreamfrom the aperture stop; wherein the refractive lens elements in thefirst and second groups of refractive lens elements that are immediatelyadjacent to the aperture stop are fabricated using optical materialshaving a negligible susceptibility to thermal stress birefringence ascharacterized by a thermal stress birefringence metric, and the otherrefractive lens elements in the first and second groups of refractivelens elements, that are not the refractive lens elements immediatelyadjacent to the aperture stop, are fabricated using optical materialshaving at most a moderate susceptibility to thermal stress birefringenceas characterized by the thermal stress birefringence metric.
 2. Theimaging system of claim 1 wherein the thermal stress birefringencemetric includes factors relating to a coefficient of thermal expansionof the optical material, a stress optical coefficient of the opticalmaterial and a light absorption coefficient of the optical material. 3.The imaging system of claim 2 wherein the thermal stress birefringencemetric further includes at least one additional factor relating to amodulus of elasticity of the optical material, a thermal conductivity ofthe optical material, a Poisson's ratio of the optical material, anoptical power density in the optical material, a thickness of therefractive lens element or a clear aperture size of the refractive lenselement.
 4. The imaging system of claim 2 wherein the thermal stressbirefringence metric is given by:M ₁′=ρκα. where ρ is the coefficient of thermal expansion, κ is thestress optical coefficient, and α is the light absorption coefficient.5. The imaging system of claim 2 wherein the thermal stressbirefringence metric is given by:M ₁ =ρκαE/(K·(1−μ)). where ρ is the coefficient of thermal expansion, κis the stress optical coefficient, α is the light absorptioncoefficient, E is the modulus of elasticity, K is the thermalconductivity, and μ is Poisson's ratio.
 6. The imaging system of claim 5wherein the optical materials having a negligible susceptibility tothermal stress birefringence satisfy the condition that M₁≦0.1×10⁻⁶ W⁻¹and the optical materials having at most a moderate susceptibility tothermal stress birefringence satisfy the condition that M₁≦1.60×10⁻⁶W⁻¹.
 7. The imaging system of claim 6 where all of the refractive lenselements satisfy the condition that M₁≦0.80×10⁻⁶ W⁻¹.
 8. The imagingsystem of claim 6 where at least one of the refractive lens elements isfabricated using an optical material where 0.8×10⁻⁶ W⁻¹≦M₁≦1.60×10⁻⁶W⁻¹.
 9. The imaging system of claim 2 wherein the thermal stressbirefringence metric is given by:M ₂ =I ₀ LρκαE/(K·(1−μ). where I₀ is the optical power density in theoptical material, L is the thickness of the refractive lens element, ρis the coefficient of thermal expansion, κ is the stress opticalcoefficient, α is the light absorption coefficient, E is the modulus ofelasticity, K is the thermal conductivity, and μ is Poisson's ratio. 10.The imaging system of claim 1 where the optical materials used tofabricate the refractive lens elements are also selected for propertiesthat relate to achieving adequate image quality performance, includingrefractive index and chromatic dispersion properties.
 11. The imagingsystem of claim 1 wherein one or more surfaces of one or more of therefractive lens elements or the reflective optical elements have anaspheric surface profile.
 12. The imaging system of claim 1 wherein thereflective optical system includes at least two concentricallypositioned reflective optical elements arranged in an Offnerconfiguration.
 13. The imaging optics of claim 1 wherein the reflectiveoptical system include at least two reflective optical elements arrangedin a telescopic imaging configuration.
 14. The imaging system of claim 1wherein the reflective optical system includes one or more off-axisreflective optical elements.
 15. The imaging system of claim 1 whereinthe reflective optical system has no central obscuration.
 16. Theimaging system of claim 1 wherein the reflective optical system includesat least one reflective optical element having an aspheric shapeprofile.
 17. The imaging system of claim 16 wherein the aspheric shapeprofile is an elliptical shape profile, a hyperbolic shape profile, aparabolic shape profile, or a free-form shape profile.
 18. The imagingsystem of claim 1 wherein the at least some of the optical materialsused to fabricate the refractive lens elements are glasses or polymers.